Why do some people stop using online forums?

Changing technology and cultural shifts online and off. The move to phones and tablets for many if not most tends to drive quick and short responses and is more a device for forwarding of memes and conversations. As 85603 notes the imported from Yankland culture war stuff has also divided people more, or at least more obviously so. That in turn and human nature in general tends to prefer places where they can hang out with generally similar minded people, so we have the increase in so called "echo chambers" where different opinions are downvoted to oblivion or never invited in the first place.

I say human nature because the rise of the interwebs was initially mostly populated by nerds and there was that barrier. I've found nerdy folks in general tend to be brighter than average, more open to different opinions and more into rules of conduct. They do tend to be more "left wing" too, but that's part of the more open to different opinions as a general thing. Like any new arena it starts small and with some general mutual agreement about the rules coming soon after. As more and more people show up, more and more internal groups form and look to mark themselves out. Kinda like if you open up a club and 20 people show up at first they'll tend to hang out together, when the numbers hit a 1000 you get far more groups forming. I reckon the above is a lot of it anyway.

Add in generations coming along who seek different outlets for conversation and attention and the ageing demographic in here. As people age they tend to get very set in the thoughts and philosophies they have decided fit(doubly so for men in my experience) so they're less up for debate in general. Younger people, again in general are far more fluid on this score, more engaged, less rigid, more open.