Cognitive Dissonance. Twitter was popular, so they went all-in and now depend on it's reach for their audience; so now twitter being not popular, or a den of nazis, or having wildly inflated engagement numbers, or actually hurting the reputation of those still there - that's unthinkable. It's literally impossible to contemplate, as that would involve admitting they got duped, and don't *really* have a million followers, and are supporting thugs, etc etc.
(Insert image of "it's fine" dog sitting in burning building.)