I think the app part is the biggest factor. I’m using Jerboa, and while it’s still good and useful, it definitely has some bugs (crashing when uploading photos has been annoying). I’m still sticking with it, but I can see a more average internet user getting frustrated with that or with the somewhat weird sign-up process and learning how federation-y stuff works.

It’d be sick to have a standard “this is how you sign up” tutorial built into each app!