I'd imagine it's due to such forums not having a big incentive to get their userbase oxigenated, as you'd need to create an account and invest on it, but at the risk of ignoring and eventually even dropping the social media you're already familiar with. And aggravating it, as userbase shrinks, certain behaviors cause more and more people to drop, leaving only those that are ok or that even replicate this behavior, causing a vicious cycle.

From what I observed, when forums start going that path, if they're small enough, they die from irrelevance. And if a given forum is big enough or is kept forcefully alive, they become some Karachay lake-style radioactive landfill.