Largely because the internet before the mid-2000s was a fairly decent place to be. Some of those old forums are still running, and are still way more intelligent than reddit.
But I would guess a large percentage succumbed to attacks from spammers, ddos, seo stuffers, etc, until the admins decided it wasn't worth it. Oh almost all of them had their password db hacked at some point, whether they told you or not, which pushes people to centralized services.
You say forums before the mid-2000s were pretty cool, but Something Awful is still going and has always been $10.
I really don't want to be the product anymore. I would gladly pay for a good service with reasonable moderation.
I'm paying for Discord right now, even.
In practice in the present its understood that this is an uneconomical idea
Maybe its free APIs that are a zero interest rate phenomenom
Forums before the mid-2000s were pretty cool.