Computers were still more niche, more complex and required more effort to use which acted as a built-in filter; if you invest all this effort into getting onto the forum to participate there's a higher chance that you legitimately want to contribute constructively.
Nowadays, every idiot out there has access to an internet browser and already has a social media account (and can use it to log into most other websites which implement social login for the sake of growth) and start spewing bullshit.
I have noticed a similar trend when it comes to online PC gaming. I used to play a shooter game (Crysis) back in ~2008 and the atmosphere on servers was always great; the chat was respectful, there were actual discussions happening in-game and I haven't seen any disrespect, rage or anger. Nowadays I play Battlefield and the chat is mostly empty, only interrupted by insults and the occasional server info message. Console gaming introduces voice chat and seems dominated by kids swearing their lungs out. I guess back in the day the cost of a machine capable to run these games acted as a filter, where as nowadays everyone has access to them, even those that shouldn't.