A lot of the old forum applications still exist. A lot of old school forums are actually still around. The problem with old school forums compared to Reddit or Lemmy is that it’s just so much easier to get people to join a subreddit or a Lemmy community. If someone discovers a subreddit, all they need to do is go there and start posting. Creating a new account usually isn’t that hard but it’s still a considerable hurdle for someone who is considering joining.
Old school forums are simply too decentralized. It’s a lot easier to get reddit.com to come up in search results then it is for some relatively obscure forum.