Part of it is the change in inactivity policy When people had to log in to the forum, they often stuck around for a few minutes (and some folks thought they HAD to post). Another part was that there were, once upon a time, more obvious time-wasters here, what with the games and argumentation section and all. And there was always the faint hope that the credit system (which rarely worked properly) might provide some kind of value at some point. These days you don't even need a minimum post count to participate in the Free Hosting forum, and there's a bit of an "infinity" problem looking onward towards, say, the Community Advocate level when you're sitting at a 30-post count.
A bigger part of it, though, was that there was a core of people who did most of the chattering. This is mostly a hobby world; people get busy, get older, or find that their websites need serious hosting and serious admin time. (Let's face it, if you're running a site that's actually important to you, where uptime and response times mean something to your bottom line, it's probably best not to run it on a server with a few thousand badly-bloated Drupal or Joomla installs, and where a newsletter is almost completely out of the question.) All you need to do is lose a few of the friendlier folks, and things die down a lot.
If it's any consolation, there are always a bunch of self-appointed experts in the SEO area trying to sell outdated and mis-informed services to one another. They can be fun to play with.
