I’ll speak about my experience. Bluesky feels like an upgrade to Twitter. There are many algorithmic feeds to choose from, and it’s easy to discover people to follow. Mastodon, on the other hand, is a straight downgrade from Bluesky because it is lacking in those features.
I imagine a lot of people leaving Twitter feel similarly. They don’t care much about privacy or federation. Bluesky just works, and that’s what matters to them.