I've gotten the impression Eugen has a very clear idea of what he wants Mastodon to be and isn't shy about rejecting features that he doesn't think serve that goal. Those forks are the model of good open source. Other people have strong ideas about how it should be, couldn't convince the main project's owner, and moved on to create their own version that suits their vision.
It's great. This is how it should be. And it's a huge improvement over open source projects that languish for years for lack of strong leadership with no one forking it because there's no one to say no to features. As long as the possibility exists, people are wary of forking. Remove the possibility, and people can feel confident doing their own take.
As for Mastodon, as far as I know all the major forks are maintained by experienced developers who incorporate security updates.