I don’t think this is the exact cause for the situation, but having more book related forks would probably just do harm by splitting up the audience. The book reading trackers are absolutely dominated by Goodreads, and any alternative desperately needs as much user concentration as possible.

I don’t think you can correlate the number of readers to the number of book instances or whatever they’re called. Most people (myself included) probably just use Goodreads, and BookWyrm is probably a good enough alternative that there’s no need to spin up another.

Edit: according to this there’s a lot of instances: https://joinbookwyrm.com/instances/

Most people I know who read a lot use StoryGraph, and mostly for personal tracking, not as a social app.