I think this is because old school message boards had a sense of community between existing users and there wasn't a large influx of users at any single point.

I think this is because old school message boards had a sense of community between existing users and there wasn't a large influx of users at any single point. If one person comes in and starts running amok it's easy to just ignore them, tell them off or ban them. But now there's less persistent forums that people are a part of, so there's a lack of community standards that people just naturally gravitate towards. There's no overall community between, for example, people who comment on youtube, so any youtube comment section is just whoever happens to stumble across it.