Because most of the communities are on those 2 instances. In terms of userbase world does have the most users, but they're the exception as other "big" instances are more evenly split.
So instead of a single instance going down we have 2 main instances that need to go down, and even if they go down we'd have lemmee, shitjustworks, lemmyca and blahaj (and I guess also lemmynsfw) communities feeding the feed.
Purely anecdotal but… the 3 stupidest people I’ve ever met online were all from .world - and there seems to be a propensity for deleting comments that go against their world view. Only knowingly interacted with one .ml poster and they were perfectly decent.
I know, seems wild… To clarify the statement I should have said “interacted with” as opposed to “met”. I’m not on many platforms though - no FB, insta, bebo, friends reunited, the old bird site etc. I used to interact on another site, forget what it was called, maybe Robbit or something like that. I’m on Mastodon but I have a different personality there and don’t use it for posting anyway.
I have a feeling that a few/many/lot of .world users have a very simplistic and binary view of the world where they can’t see that some issues are multifaceted and you can’t just throw out blanket statements. There also seems to be an issue with some people where they don’t/can’t differentiate between opinions versus facts. Opinions are personal whilst facts are (hopefully) empirical.
They have "power users" that mod multiples of the largest communities and will delete comments of they disagree with your opinion, citing "civility" or other nonsense, even when you're repeating back exactly what was said to you. So, yeah, abusive moderation.
I'm actually a big fan of the misinformation moderating they're doing on world. It's very Lemmy.ml of them in trying to maintain their political correctness.