I've suspected this for years and having researched the history of lawncare a bit I think my theory holds water.

Thesis: After World War 2 the US had 2 generations of men returning home from war with severe undiagnosed trauma. WW1 showed how terrible the effects of trauma or shellshock can be, the powers that be were terrified at the thought of 10 million men coming back to normal lives and having so much time on their hands. The way we processed trauma back then was awful, and the adrenaline withdraw, lack of excitement, and other issues about returning from combat were barely being explored.

Sunday there's church and hopefully family obligations that keep them from being idle.
Monday - Friday they work long hours and only have a little time in the evening, but what about Saturday? Idle hands are the devils playground, and we need to give these men something to do other than sit with their thoughts the whole weekend. This is where 40's and 50's era suburban living comes in...

Backyards, barbecues, lawn darts, picnic tables, picket fences, shrubs, home repairs, and of course, the lawn. Obviously these things existed to some extent before, but now there's much more labor to be done, and it's expected of everyone. When people did have greenspace on their property before, it was allowed to be natural and the balance was kept with native species. Now everyone is expected to have a plot of meticulously kept grass that has to be 1 of 5 species and has a narrow window of how tall it's allowed to be. Other plant species aren't allowed unless they're organized off to the side in some mulch or a garden.

That expectation is one of the biggest pieces of evidence, suddenly there was a big social pressure that wasn't there before. Not only do people have this obligation, but they police eachother. Your neighbors probably won't report your car having an expired sticker, or oil in your driveway, or overfilling your trash bins, but let your lawn get over 5 inches and see how quickly someone intervenes. Right from the beginning the lawn became a holy obligation that people are harshly judged for ignoring.

Loose Evidence:
Comment Update: "This isnt a conspiracy one of the original selling points for funding development of house with lawns was the idea that if working class people spent time maintaining lawns thats time they couldn't spend reading communist theory"

People started policing eachother about it right away and it became an expectation.

Compliance is tied to your value, only derelicts and bums have messy lawns. (Multiple neighbors have told me I drive too nice of a car to have a lawn like mine)

Timing. It makes sense that consumerism would kick up and generate all of these new products and things for men to do. But the fact that they were an obligation and not much of a choice seems suspicious, especially with all of the returning soldiers.

Labor required. I can't think of another example where something as labor intensive became an expectation, and not just something to opt in to. The attitude is "I don't care if it takes a few hours, tires you out, or if you're too old to maintain your lawn. You better fix the problem"

Increased Complexity. As time has gone on there are more expectations added, now you're obligated to do something with your clippings, or at least keep them off the sidewalk or away from the street. You're expected to keep the cracks in your sidewalk clean, expected to edge your lawn, or use weed killer, and it better not be brown it better be a deep lush shade of green.

Effect. If it was a psyop it was super effective. Lawncare is a cornerstone of American Male culture, more products have been developed to soak up your weekend time. Home Depot or Lowes constantly ask 'what are you doing in the backyard this weekend?' as if you're meant to constantly have a backyard project going on. HOA's exist with some pretty insane rules about lawncare and police their members much more actively. Also the landscaping/hardscaping industry. People who can afford it are able to pay someone else to do that work because it's unpleasant but expected of them.

So maybe it was a psy-op to give returning soldiers something to do with their hands on Saturdays...
Or maybe I just hate mowing the lawn.