Dedicated forums for particular topics were generally tiny. A forum with 10k+ people was a sizable forum, and not many where that large. Small communities got very little attention from spammers, and while they had their own trolls giving problems they were much less rarely subjected general trolls unless something like 4chan or whatever came before it picked up on them.

The internet is also huge and has zero degrees of separation in most places. I could post a link to a tiny forum here and 1000 people may see it. I could post it to reddit subthread and a hundred thousand people could see it. Or I could get a front page post on Reddit and a million+ people could see it. And with it tons of trolls, spam, and general idiots will follow and quality goes to trash.

This barely has anything to do with Reddit/Facebook itself. It's the power of network effects congregating people. Facebook was always going to happen, maybe in a slightly different flavor, or slightly different form, but once we got the computing capability to have millions of people at one place at once time, someone was always going to build it.

Video game producers are starting up fan discords that also have the troubleshooting/bug reports channels for the game on them. Video game mod communities are moving to discord.

Its going to really suck when a few years go by and a company closes or moves onto a new game and shuts down their discord. A lot of community sourced info is going to be flushed down the drain if people don't archive it.

Piss off the admin and you might be insta-banned with no recourse. You'd even lose connections to the other users on the forum and would have no way to reach them. You didn't have the option of opening a competing forum where all of the old users already had access like you do with opening a new sub. You'd also lose access to the posts themselves and any content you found useful for the private forums.

You were reliant on the admin to keep the site running. Most had extensive downtime. Things would often break. The admins would have to run charity fund raisers to keep things up and many would threaten to take everything with them if people didn't pay up.

Forum administration was terrible, but there was a far healthier ecosystem full of alternatives that no longer exists. Remember, SomethingAwful's overmoderation was what popularized imageboards for westerners, and Digg's failings were what popularized Reddit. There is no equivalent today.