Well, best thing to counter that is to be the change you want to see. Start the community for the niche you miss the most/know the most about. Post something daily for two weeks + do a call for mods, and before you know you'll have at least some activity. I (helped) to bootstrap !openstreetmap@lemmy.ml this way, and the community has about 1 top level post per day on average now. Not a lot, but a good start.