Because people, even leftists, want the bad company to be replaced by a good company.
Why replace horrible company with bad company run by literally THE guy that let the prior company become horrible instead of replacing it with good non-profit network?
FOMO. More celebrities are on Bluesky than Mastodon and people don’t care enough about open protocols and so forth to forego that. If Taylor Swift was only on some Fediverse-enabled platform and nothing else, people would come here in droves. Taylor Swift does not post on Mastodon so people don’t want an account here. Replace Taylor Swift for anyone of any sort of popularity and ask the same questions.
I do wonder who the most famous person in the world is that exclusively posts on a Fediverse-platform. It could very well be Eugen Rochko, who probably has about a 0.05% name recognition throughout the world.