I think those who are not satisfied with the direction Reddit is going are already here and found a new home. Maybe some will come here after the thrid-party-apps cease to work (if they aren't here already).

The vast majority (the "bulk of Reddit") doesn't bother. As long as they can use Reddit the way they use to they will not leave. It possibly has to do with the fact that these users are not tech-savvy enough in general. I don't mean it in a negative way. They just don't care because they just focus on other things. And with this in mind these users probably checked out Lemmy already but because the buttons are green and their favorite subreddit is not here yet, it is not appealing to them. So they rather stay on reddit.