Because the people who run the majority of instances are leftists. And despite my many grievances with marxist-leninist ideology and particularly with stalinism, I'd rather talk with tankies who at least claim to respect me as a minority than nazis who actively wish death upon me.
Leftism should be tolerated because leftism advocates for the rights of the workers. Leftism is the way the world can and should be. Whichever flavor of it, they're all better than fascism. And wherever fascists will gather their sole purpose is to propagate hatred against minorities. Leftists are overwhelming welcoming and accepting of minorities. They are no threat to the acceptance of marginalized peoples.