TL;DR - It's not THE multiverse, it's a diseased branch of the larger multiverse.

1) All of the Starfield multiverses I've seen or read about starts with the Mars Find. Consequently, no part of the multiverse seems to NOT include the Mars Find.

I find that suspicious. (Ha! That was a pun...)

2) All of the Starfield multiverses I've seen or read about end with jumping into ANOTHER Mars-aligned branch of the overall Multiverse. (Sarah? Are you there?! Wiggle your leaves if you can hear me!) There's no Simpsons Halloween Special on Time Travel happening.

That makes, at least in MY mind, the Mars Find an anchor point in this branch of reality.
If Mars happens, you end up on this branch. Do not pass 'Go'. Do not collect $200

3) Within the Mars Branch of the Multiverse, there is looping happening with Starborne. Why? For the same troubling reason as the Mars Find: Because it grants you powers you probably shouldn't have. And you get to fight other loopers (starborne) over those powers.

Therefore!

The Multiverse as a whole isn't cool with what happens down the Mars Find Branch.

It can't be stopped, but it can be isolated from the rest of the multiverse, and contained within itself, and anyone who figures out to grasp the power spends the rest of their existence fighting other power-grabbing loopers.

You don't get to graduate into a part of the universe that doesn't have Artifacts.

That makes the entire Mars Find Branch just one big multiverse Containment Field.
Everything tainted by the Mars Find just gets.... separated and trimmed.

Battle Royale in spaaaaaace!