I think this to be a consequence of forums being isolated from each other, while communities in Discord/Reddit are connected.

If you want to talk about N different topics in forums you need to register N times, check N separated feeds, and establish your presence N times. This quickly piles up, and while you might stick to one or two forums you’ll eventually ditch all the others.

In the meantime Discord/Reddit streamline the process. Even if I spend most of my time in r/apples, I’m already able to post in r/bananas, r/cherries, r/durians etc. If I were to subscribe to all of those, I’d see new content from them in a single feed. And people from those comms will see my activity in r/apples and know “hey, this is not a troll”, so my presence is already half-established.

The problem is that Discord/Reddit have a single point of failure: the administration can enshittify the whole thing. And they did. Then we get something like the Fediverse picking the best bits of forums (self-governance, no single point of failure) and Discord/Reddit (less isolation).