So before I start I want to say everything is my **opinion and interpretation.**
I am no way **speaking for everyone**, every person is unique, even if the general crowd seems to be such a way, doesn't mean you or the person behind you or your friends are like that.
So don't get offended and think I'm labeling you personally please.

With that out the way I'll just get right into it.
I'm going to just get right into it and speak about the North American Players and by extension European as I think NA mindset is the most complex compared to japanese...Also I want to point out I never raided on EU but I believe they have the same mindset as NA as many NA statics I've joined had many europeans in it and many players I meet in NA are European.
Now one of the reasons I believe NA despite having a larger player base than Japan, NA tends to have lower savage clears rates.
This is because NA prefers and values their uniqueness and individualism. What I mean by this is very often, each unique static is going to have their very own unique way of solving a mechanic. This is nice because each group is different so you'll learn different ways to do the fight but this is very bad for the overall NA raid community. Because what this does is less and less people will join a PF due to widely different strats they've learned.

How many times have you joined a PF in NA only for a person to leave or wipe because they say "Oh you guys do this way different than my static" or "This strat is stupid". With NA there are so many different strategies people do and though this isn't a bad thing overall, as it shows the fight can be solved in many ways. It IS bad thing if you want people to consistently clear in PF.
One group in PF would say we do BPOG another group would say we do Lines.
This makes players less likely to join pf and just join statics or just not raid at all because everybody is doing something completely different.

NA tends to be very proud of their homebrew strats so there is a big unwillingness to change it if you join statics that do something different because it's how they learned it.

Another thing is NA very much prefers personal performance over just efficiency/playing safe.
Often NA unspoken motto is the faster the boss dies, the faster we clear.
That's totally fine, but what that means is many MANY NA strats are uptime strats rather than "safe" strats and very often NA party finder adopts uptime strats as the PF strat and that's not good because uptime strats are very often more dangerous to pull off and really high chance of someone dying. Since many NA players value their personal performance a lot more, they tend to greed more and overall play more reckless to get that extra GCD. There are even times where people just outright refuse to limit break because it'll hurt their parse even if it means wiping on a clear run and not a reclear and since people usually want their good parse - this can also make NA overall more hostile as people will get angry if you kill them or they'll quit and not raise back up and just rather wipe everybody to start over so they don't get the dreaded grey parse.
Healers letting their co-healer doing all the healing so they can get a pink healer dps parse and so on.

Safe strats are often looked down upon on NA.. extremely so.
If the melee has to lose more than 2 GCD, that strat is in the trash regardless if you don't need the dps..and so are using strat macros.
I remember I posted a macro I gotten from my main server and I was laughed at and people just ignored it or nit picked it as it was some kind of alien writing since they couldn't understand the boxes and Letter placement. This wasn't a one time thing as it happened more than once and I quite got the picture that this isn't a thing NA likes. People just laughed and say "LMAO look at miss macro over here" or just simply pretend I didn't even post anything or just immediately dismiss it.
Outside raid strat problems.
NA very much has a Voice Chat Culture. Now I know this is common on NA but I believe just because it's NA culture to mostly VC - I think this scares people off from joining statics due to social anxiety or just being shy.
Though some groups say you don't have to speak, most groups say you HAVE to speak. Now VC is normal for statics but the problem is this extended into PF as well and what this tells a normal player is they're probably going to adjust strats a lot or other things. VC requirements are more common if you're doing your savage weeklies soon as reset happens on NA.
Now NA DOES sometimes have PF "strats" but there are still more than one and usually often it changes- and on top of it changing, its always uptime strats and as I brought up before. Depending on the fight, its usually risky and has a high chance of failure.
I remember when I was pf'ing p8s, there were 3 or more different strats for PF.

Other than raid specific issue is NA players are -very for sure of themselves- when they don't want to do something.
Someone could be decent at the game, only doing extreme battles but won't even touch savage or even give it a shot and test the waters, they just convince themselves they can't do it and this is an issue with many NA players.
My opinion is this lack of confidence is due to various different strategies they might have to learn, social/performance anxiety, just low confidence such as thinking their parses are bad and more, just a huge overall defeatism attitude.
Another reason is in North America - the raid community still has the bad reputation of being toxic and hostile, this makes new raiders or rather, "potential" raiders not attempt to raid.
Making the clear rates for NA even smaller since fewer people attempt even though many MANY people are actually capable of clearing as I played with fantastic players that have zero confidence in themselves and they rather just stick to extreme battles.

Now for the Japanese Raid Community.
Japanese strats are very...."uniformed" or "Static". Which could be boring for people that like to do different things. We prefer strats that are efficient, safe and consistent that anybody can do and allow you to do the required dps to clear.
When you join a group - you'll know where to stand, what to do, where to go and know exactly how to handle the mechanics as your role.
Because strats are the same, everybody just studies one guide without worrying if group B will be different from group A.
Someone will almost always use a macro of the strategy as well, so you'll always have a reminder and this reduces the need to discuss and talk beyond setting partners and light parties. Japanese players use macros to explain strategies rather than using voice (PF) or just manually typing it out like NA/EU(Unless you have a question of course). This consistency of a singular strat is one of the biggest reasons why the Japanese clear rate is so much higher than NA for savage.

Strats are always the same, that is unless you join a high-end static or one that focus on parsing/speed killing.
But when it comes to playing with randoms and PF it's always the same no matter what. Even statics use the same strats and rarely do they have their own unique ones, that is unless you're doing week 1 savage and their is no established strategy yet but even those people change their strats to the norm.
Japanese players are less focused on personal performance(parsing and playing recklessly, greeding) and just do what's necessary to clear which means people actually limit break even when it's not necessary and sticking to the strat which usually/ might have downtime depending on the fight for melee players.
This makes it so on average - JP players normally are grey or green percentile but unlike on NA, people very very rarely upload pf clears/clears with randoms to fflogs and if they do there is a high chance they'll hide their character.
Japanese players tend to be more respectful of people privacy so even if someone got a orange or pink it's likely they won't even upload it to fflogs still.

Another reason is Japanese are a lot more willing to try endgame battle content even if they don't think they can do it or will enjoy it. But because it's endgame, they feel they might as well try it even if they don't succeed because many players feel obligated to at least do it. More often than not most people actually do end up clearing and this makes them do the next fight and so on.
So just based on that alone, we have more people trying savage - because even if they think they can't succeed, they give it a shot anyway because they feel because it's endgame content, they should at least do it/try it.
This is also a big difference from NA players that wouldn't give what they don't like a time of day as NA/EU value their time a lot and if it's something they don't like like, they won't waste their time on it no matter if its endgame content or not.
So this makes it so the clear rate from Japanese players are higher as more people actively try to clear savage even if they don't think they can, most end up surprising themselves but this is also helped by the fact strategies are the same , so they can study one strat, unlike NA that unfortunately has several you have to learn because each group might be different.

If you want to know how much we value a single strat for the data center..
When data center visit happened, there was huge drama because other data centers were trying to bring their strategies in, which was different from ours.
It got so bad that mana natives started /telling non natives to leave and also people were writing party finder descriptions such as "Mana players only" People were getting worried we'll end up having people with various different strategies, making things confusing for everybody and make things hostile as people were fighting on which strategy to do, it got real bad and reminded me of NA, only more passive aggressive.
Though this has died down a bit and people are learning to just follow mana strategies, it's still an issue we have right now.

But I'm going off topic as that has nothing to do with the clear rates.
Anyway to wrap it up.

Japan's consistency and willingness on using a single agreed upon strategy that gives people a single guide to study and understand - more people willing to give savage a shot and the general lack of hostility (to your face, because we'll take crap behind your back) and putting clearing above their own gain and personal glory (parsing, greeding) makes it so our raiding scene is generally more approachable and making the success rate in clearing extremely high.

This is a big contrast compared to NA that favors their own individualism.
I believe NA culture of individualism is one of the biggest reasons as to why I believe it's impossible for NA to ever reach/equal or surpass Japan clear rates for savage.
This makes it so many people just refuse to do the strategy they didn't come up with or ones they didn't learn. Many players want to do THEIR strat and not what the whole community is doing or trying to do. This on top of many players unwillingness of giving raiding a shot due to many reasons, such as just lack of confidence, too many various strategies to learn, the stigma of toxic raiding, social anxiety of voice chatting, defeatism and more.
I don't believe when it comes to savage raiding, NA would ever collectively do a single strategy across the entire data center due to NA culture of individualism and wanting to do their own thing apart from everybody else, on top of NA players preference for personal performance due to NA raiding culture heavily using FFLOGS to judge other players on anything. This isn't to say Japan doesn't use FFLOGS to judge you but it's less so compared to NA who uses it to judge your entire character and performance and if they should listen to you or not.
Japan is less likely to judge you harshly if you seem to be a pug player.

When I think of NA/EU and JP Raid Community It goes like this.
NA/EU: **My** way or the highway (a single group)
JP: *Our*" way or the highway (entire data center)

With all of that...to finish this thread
Japanese are not better players skill wise...if that was case then every world first would be japanese teams.
Every region has amazing players and every world first is almost equal between regions.
Japan having a vastly higher savage rates doesn't make us better players, its just a huge difference in mindsets and cultural differences.

TL;DR

Americans beliefs and individualism prevents them from being as successful in PF Savage raiding due to wanting things to just be their way/ along with their need for only caring about their own personal performance over just clearing and also Americans have bigger unwillingness in trying content if it doesn't interest them which makes their pool of raiders smaller and less clear rates than Japanese due to various of different strategies and also strategies that prefer uptime which are generally more risky and harder than playing safe which makes pf much less successful.