I only skimmed it but didn’t see this: part of the issue was that forums were prone to be shut down completely, or lose user data when migrating to a different forum system. It made it hard for you to have a repository of knowledge on a topic/ hobby when it could just disappear. Reddit/ Facebook/ discord promised an easier way for organizers to set up their communities, and community members had more trust that these new communities weren’t going to go anywhere.