Mastodon is nice an all (I switched from twitter to mastodon 2 months ago and have really enjoyed it) but some people just want twitter 2 and not necessarily a federated service where you have to worry about what instances are blocked, who can see what, etc. Also the fact that some people want twitter to interact with celebrities and content creators, at least from those I follow almost all the ones that mentioned moving are going to bluesky.
Those are the two reasons I can think of on why not just moving to mastodon, I love mastodon but for me its more about interacting with people in a way different from what rwddit was fue to the lack of feed and algorithms to show you content, it is all chronological be it an instance you look at, the tags, etc.
And mastodon lowkey lacks being more approachablr right now, for the average user choosing an instance is a difficult task, there are many good videos explaining it and all, but joining mastodon for a lot of people takes effort since it involves learning about the fediverse and that is a bareier of entry that prevents the mass appeal twitter has.
I do feel this issues will solve themselves once more people start using it and once it gets more big celebrities (not just the tech related ones), but it is understandable why some people want a different thing.