Small forums worked because a lot of people soaked up the cost of running a little web page on a box that wasn't very expensive per year. A lot of them eventually, decided they didn't want to spend the money or time and hand off to a new maintainer.
The new maintainer looks to (reasonably) expand the community and embrace new technologies, e.g. live chat, video. This creates 10x the work at least and eventually they give up and pack it in.
Now, unfortunately the work to take over the community on it's current state is much larger than the original site and so it eventually dies without someone wanting to work a full time job without the pay...
I miss the optimistic lose communities of the 90s when I was growing up. Not to mention the communities on topics which are now no longer of interest to a wide audience. The amount of organic search needed to rediscover any new forums now is also unfortunately a new barrier to entry there never used to be :(