Why aren't they as popular now? Old forums are naturally losing users - HeroesWorld, for example, is first and foremost a forum about Heroes of Might and Magic, and few people are interested in that nowadays, so there's no reason to google such a site (plus, finding something like that in search results is probably harder now), while old users are simply worn down by life and gradually drift away. At the same time, new forums for fresh topics are probably barely being created, because why bother? You make a thematic group somewhere and that's it, no need to go through the hassle. So that's how it ends up.
By the way, I'm tired of the topic of hating on social networks and online communication in general. I remember I had a very unhealthy atmosphere at school, plus constant scandals at home, no IRL friends, but at least I could log onto HeroesWorld (and to a lesser extent other places too) any day and talk to someone, do something interesting and creative, etc. I think in those years it really helped me develop communication skills, imagination, and many other things.