I'd expect that it's because if you have some new forum without (as many) restrictions, you get both: - people who disagree with the restrictions of other plac

I'd expect that it's because if you have some new forum without (as many) restrictions, you get both: - people who disagree with the restrictions of other places on principle and want a freer alternative - people who can't say what they want to anywhere else, because it's generally disliked and most people probably do not want to read stuff from the second group.