We lost the “local” based internet groups and we only really have “subject” based groupings anymore.
Spammers killed many a forum because it became a full time effort by admins and moderators to remove them. Automated tools, especially early tools, had limited effectiveness. People running a site as a hobby don't want to spend their days removing spam comments.
Assholes also (for many reasons) will DDoS forums. This is another administrative headache for site owners if not a financial headache.
The same driving force saw blogs remove comments for centralized solutions like discus or abandon self-hosted blogs for Medium and the like.
While Facebook and reddit have plenty of spam issues the "owners" and mods of a group/subreddit are different from the site admins. There's a lot less financial risk for someone running a group there than their own server. They also let a wider swath of people run a group rather than only those that are interested in a niche topic, have the technical skills to effectively run a forums, and the money to pay for it all.
I miss bbses. I resisted the internet switch for as long as possible. I remember the last c64 release..wasn't it monsters of mayham.. the graphics were amazing and the gameplay fast. Sort of like a mario world.
BBS could be elite where you needed references. It could be local family focuses with no swearing. There was a network of bbses who would share posts.. that was cool.
The best thing about bbses. As a SysOp I could start chatting with any user. The person logging in actual used your computer so in real time you could watch what the user was doing. Websites lost that.