I keep a forum going that's been up since 2007 for hobbyists. I keep it going because of the priceless historical information on it, not the present quality of discussion.
My sense is that people have become less literate, less polite, and less patient.
Again, this is for a simple hobby so there aren't necessarily many intellectuals in this hobby, but 10+ years ago we had many lively, intelligent, long discussions.
What happens is you start getting more and more users who barge in, hijack threads, and do not contribute anything useful.
They can also be abrasive, which spooks the old timers from prior generations who were used to intelligent, polite discussion where they agree to disagree.
Eventually the quality content thins out, and there's a sort of Reddit effect where you get one solid reply for every ten useless ones.
These days I find most of the new users are just looking to find out what their collection is worth.
As those new users fill up screen space with vacuous content, the old timers are fading away either through attrition or illness or death.
On the technical side, you're constantly battling bots because your domain age and reputation is targeted by spammers.
Just hazarding a guess, but don't discount ease of setting up and maintaining a subreddit, compared to a forum. And the ease joining and participating in a subreddit (which is essentially zero effort if you already have a reddit account). Instead of signing up, giving out your email to another potential spam source, etc., etc..
The ones I was apart of sited Facebook, and most people moving there, as the reason. I think some of it was also getting older and changing priorities in life.
With one of them I tried making a group on Facebook to see if that would keep the crew together with the same vibe, but it didn’t work. Though I’m still in contact with a few people.
Subreddits have dropped off too. I assumed the migration was to discord or tiktok or other plublic squares.
Specialized forum content, discussion and community suffered with the VerticalScope rollup that borged them all a few years back. They immediately upgraded the forum software to be phone app compatible (more eyeballs) and forum quality died soon after. No one is have good discussion, arguments, stories or history while phone posting on the can or in line at the DMV. It is all repetitive drive-by comments or questions that could be answered by a quick search of the forum so it is short attention span theatre now. I saw this across multiple forums that got rolled up by VS.
tldr; Vertical Scope and phone app use.
https://www.verticalscope.com/our-solutions/be-the-solution/