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.
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.