Part of it I think is that old-school forums tended to have tighter-knit communities that weren’t as prone to shifting around. People had to find the forum in the first place and then register a new account to post, which may sound trivial but presents enough friction to filter a lot of people (including many low-quality posters) out.

Contrast this to Reddit: its search and web searches instantly surface subreddits and one account gets you access to the majority of the site. Friction is nearly nonexistent.

I think forums were also less prone to power-tripping moderation. It was still a problem, but there’s not but so big-headed somebody can get about being a mod for some backwoods niche topic forum that will never get all that big.

Finally, some of it is owed to how forums were dominant during a time when the internet was primarily populated by the sorts of people who owned computers then: academics, nerds, and open-minded early adopters. The dynamic was just different. It’s also the part that’d be most difficult to reproduce today.