I think there's a multitude of different reasons why, much like everything. Two particular reasons that stick out in my head are a relatively small community and lack of purity spiral, at the moment.

A small community will, almost always, be a more civil community. When more people come in, the chances of hostile elements entering increases. Which is why when there are barriers to entry, a community often thrives in quality (gatekeep your hobbies people, for its sake).

The second one is a purity spiral. If you intend to be part of the regressive/ puritan left, it's not enough to be in support of far left socioeconomic ideologies, causes and "allies". There must be enthusiastic and ceaseless support of them, matched only by your vigorous condemnation of blasphemy. For most, this will push them away, either because of an ideological difference or a general feeling of uneasiness from the obsessive drive. But then there are those who don't have means of escaping it and "break". We're all here for differing, yet broadly similar, reasons and we don't feel the need to shout our beliefs from the rooftops and manipulate, abuse and browbeat everyone else into doing the exact same.