I'd imagine that the complaint is that they're old and don't particularly meet UX expectations in the 20s.
I do like them, and have great memories of them, they were my childhood. But I do think we're overdue for a modern forum stack. Discourse was a step in that direction, but a lot of people strongly dislike it, and it doesn't seem to have gotten the reach of others before it.
A real contender for "new forum software" would have to address the fact that Discord is its competitor, not other forum software, and it would have to have a very compelling answer for "why this instead of Discord".
I'm referring to the whole UX of it, really. The experience for mobile users, the "join community" process for new users, the experience for the community owner, not just moderation but how easy it is to start a new community. You can go and start a new Discord community in a minute. It's free and with no restrictions on the number of members.
Anyone wanting to launch new forum software needs to compete with that.
And on top of that, "just self-host" isn't enough in an era where developers don't even want to self-host their own projects anymore, let alone some third-party forum. That's another thing that has -deeply- changed in the last 20 years.
vBulletin is commercial, as is Invision Community, its primary competitor.
phpBB is pretty badly dated. It still works but it's clunky and development has been very slow. (The last major release was four years ago; everything since then has been minor maintenance.)
Well for vBulletin specifically it's that the software was bought out by a company called Internet Brands, who got rid of the original team and screwed up the last three versions of it.
vBulletin 4 is okay, but 5 and 6 are poorly coded, extremely inefficient and full of bugs even today.
Some of the original team went off to create XenForo, which has basically replaced vBulletin for most large forums now, though that itself has its own issues.
