Network effects. It's the same reason people are locked into twitter, even if they want to leave. They can't leave until the other people they depend on leave, and those people are in the same situation. A social network is a molecule, and your bonds will keep you from moving unless the entire molecule decides to move at the same time.
That's why Going Dark is an effective threat: it forces everyone to go somewhere else at the same time. Whether you want to exercise voice or exit, dealing with corporations requires organized, collective action.
Any community that isn't massive and doesn't get shown on the front page is most likely doing what the community members want. Things only go awry when it gets big enough that corporate starts to pay attention, or "normies" from the /all feed suddenly decide they are subject matter experts and have extremely important opinions.