I think it’s because leftists are, as a group, a lot less toxic and hateful. They are happy to be a tiny community chatting with each other about being a trans furry communist all day every day.
The right on the other hand are, as a group, a bunch of anti social losers who thrive on hating and complaining about stuff. That’s not a sustainable platform because it’s either a miserable experience or it gets shut down for hate speech so they migrate around and attach themselves to heathier communities and complain about those instead.