There's lots of non-blue checkers who continue to use twitter, even unmonetised too. Cos their interest(s) are there. There are plenty of niche and not so niche clusters of subgenres and activity around interests to be found on twitter which has little or no representation here.

But yeah also people don't want to start over. Like larger accounts who have a follower count larger than the entire network here. And what they post about may also not even have an audience on nostr in any significant amount to feel like a community or otherwise meaningful. Not that they wouldn't be welcomed..but more like they would feel like they're not getting any engagement on the topics they care about.