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.