Let's see, from the top down I found:
1) Far-right cesspool
2) Far-right cesspool
3) Far-right cesspool
4) Anti-trans "women's community"
5) Something actually decent
1) Far-right cesspool
2) Far-right cesspool
3) Far-right cesspool
4) Anti-trans "women's community"
5) Something actually decent
Only catch is that you have to be invited to create an account.
Keeping the community small is probably a big factor in maintaining a high signal-to-noise ratio.
There are lots of great small subreddits. Occasionally, one will become popular and it will all start to fall apart. L'enfer, c'est les autres.
> Something actually decent
Which one is it?
> Which one is it?
The list linked is ordered, so if you count down to #5, you will find mstdn.social
I'm not sure what the other possible reasons are. Due to the network effects, you'd expect people to want to use the larger social media websites by default. And theses alternatives don't seem to offer anything special (except possibly having less censorship).
Seems to me that Reddit tolerates hating specific sorts of people and not others, so if you hate the sorts of people that Reddit doesn't want you to hate on their platform, then one of these other sites will allow you to hate those folks together with others who share your hatred. While if you hate the sort of people that Reddit thinks it's okay to hate, then you can find others to hate together with, without leaving Reddit.
On the other hand, it's easy to find communities on Reddit which have no hatred towards any other groups. The same cannot be said for the top four sites on this list.