From my perspective there was this one forum I joined in 2005 it was about pro wrestling. The admins were such hardass gatekeepers going at new members who were "marks" and were unwilling to understand how pro wrestling really works + relentlessly punishing for every minor rule break. Needless to say the niche wasn't big and thru those actions the forum only lived for good 5 years.
Old members who survived that camp stayed and became loyal but as time went by new members did not join and old ones got away because life goes on. Shame. I wasn't smart enough back then to know better too that this won't work in the long run.