I'm thinking there's an inverse relationship between the size of the userbase for a social media platform, and the level of specialization of the communities. The Old Place, having millions of users, had lots of little specialized communities - r/trees, r/vaporents, r/oilpen, r/dynavap, etc.
Here on Lemmy, the userbase is smaller, so there's not enough activity on super-specialized communities, so people hang out in c/trees mostly - less specialized, but more users that way.