Due to federation: basic functionalities people are accustomed to from Twitter don't work or are difficult to use (remote block, remote reply, remote fav & repeat).

Due to architecture the user has to learn a lot before he can do basic stuff. This is true even if he stays in his "home"-instance and mimicks the silo communication of Twitter.

For a new user the advantages of federation are negligible, its disadvantages are tiresome.

GNU Social (and federation in general) is for people who share some basic concvictions (copyright, owning one's data, freedom from commercial pressure and ads, etc.) None of that is of major interest for most new users.

So, to use (or learn to use) GNU Social & federation just for idealogical reasons is simply not worth the effort. OTOH, GNU Social & federation don't provide additional functionalities that could differentiate them positively from Twitter.

... just some points ...