I didn't think about this, and it's a really good point. Around 2011 or so, we were at the inflexion point where blog comment sections and forums were losing audiences to social media.
It's true that many forums simply did not have a mobile theme, because they were set up by hobbyists who accidentally became webmasters. Not sure if vBulletin and phpBB saw the need for mobile themes. I remember that the biggest Liverpool FC forum online had a desktop only theme as recently as 2014.
That gap ended up being filled by an app called Tapatalk, which pulled in data from message board APIs in a native app setting. It was the dominant forum app because it supported all the big messageboard frameworks. And wouldn't you know it, it started filling the app with ads and subscription upsells. I can imagine many people abandoned the app, and maybe threw out the baby with the bath water by abandoning the forum as well.