Many just want to follow specific content or people, and that content/people aren't on mastodon. Since threads shares accounts with instagram and instagram is very popular (more so than twitter), the transition from twitter to threads is easier, alot of the content many want to follow may already be there.
Though, atm threads is lacking even basic features, and while it's been said that those are coming in the future, it's not a good enough alternative yet (plus not available in EU yet).
People wanting to leave twitter is primarily because of how much Musk is making twitter more and more of a pain to use (last weekend's read limits, putting existing free features behind a paywall, and such),
Not to keep the off topic train running but I think there's another important thing on top of that: ease of sign up. People TRIED to leave Twitter after it reinstated a bunch of white supremacist accounts, to places like Hive (which was shady and largely failed to work for a lot of people, the app STILL doesn't work on my phone), Mastadon/federated (the extra slightly complicated signup deterred most), Cohost which has no app because going on the app store puts your NSFW support at the whim of Google & Apple suddenly, etc.
We may scoff at things like the fediverse being "hard" to sign up for but for the overwhelming majority of social media users, it IS. They never had to deal with finding and signing up for the right forum for you, or bulletin boards, IRC, even things like when AIM/Skype/etc. all coexisted. To them signing up for something is install the app, pick user name, done. And to a degree, I agree.
I have some other qualms with the fediverse but outside of Cohost (which again has no app so it's never going to be big) it's the most reasonable option out there and it sucks they couldn't have a simpler sign up process for even a bit to onboard a lot of people. I know the idea isn't for the Mastodon server to be the default but it largely is anyway and people that care more can sign up on their own server regardless.
The main thing I use social media for is following artists (some horny, some not), and the odd news / gaming news tidbit. For a long time, that was Twitter, SO many artists had a presence on there. Now most aren't on Cohost, or Mastodon, or Bluesky (which, eh), but I see a ton of them signing up on Threads because Instagram is huge and it's so easy. It sucks because I don't know if it'll ever be in a state which I could use it for my fairly trivial usage.