Fewer barriers to entry? A forum is just a simple registration, usually with email confirmation and maybe a captcha once. Facebook wants real life personal information, blocks VPNs and nowadays I think you have to even provide phone numbers or a custom video of yourself. Discord, ON EVERY LOGIN, wants me to solve a 2 level captcha that loves to repeat itself multiple times and to do a two factor authentication while being just a bloated confusing mess of a chat. Reddit also now blocks VPNs like crazy and loves to shadowban you if you're inactive for a while (or whatever random reason they went with that I cannot think of).

They're the absolute worst possible choices and exclude countless of people.