My guess would be that to be on 2chan/4chan "back in the day", you need to be terminally online. And being terminally online is a soft prerequisite to being really good at posting interesting things online. Excellence isn't an act - it's a habit.

There was also 2ch. net that was a lot bigger, but 2ch "alumni" aren't as good. It's not just cohort, it has/had better action-reward loop than other systems.