The problem is that there isn't a singular replacement. People use Twitter for that feeling of being up to date with everything happening in the world. Until one of the alternatives can deliver that on a larger scale, Twitter will retain a considerable userbase.
I'm pretty sure the people who still use Twitter use it for the drama. Mastodon and Bluesky just aren't as spicy to them. Hell, Mastodon takes setting up to even get anything interesting.