They've existed on Freenet since before Bitcoin existed, and they function pretty well. (Usenet is arguably also a very old form of decentralized forum.) The problem is that:
 - People are too lazy to install software to use a forum.
 - People don't want to be responsible for their own moderation. There are effective ways to do it which aren't manipulable and don't require wading through endless spam, but it's still work which every user must do for themselves. 99.9% of people simply won't do it. (Freenet's FMS inspired bitcointalk.org's trust system. Imagine the forum's trust system, but for moderation instead of trust, and no DefaultTrust.)

So there is essentially no demand. If you built it on the "new hotness" ipfs (which is Freenet but much worse... and I'm not a fan of Freenet's design), then maybe you'd get a bit of attention for a while, but it'd eventually fade.