I'm guessing it's just easier to post these topics for some reason. have you noticed this, too? Do you desire a post not on the bland menu?
People often go to forums in order to form new relationships or connections with others they wouldn't otherwise run into in their daily life. They often seek acceptance, affirmation, and friendship. If the members of the forum are often at odds with them on various topics, they'll wander off because their viewpoints are shouted down.
Thus many forums become reasonably bland echo chambers - everyone pats everyone else on the back. I've stopped participating in a subset of threads where I have a distinctly different viewpoint because people here attack me rather than debate the issue. I don't post controversial threads because I already know the outcome.
Further, most forums have a "please be nice" policy, but it's enforced in a way that lively debate is discouraged. Rather than removing the offending posts (usually personal attacks), whole threads are closed under the assumption that once a thread goes bad, it's impossible to save it. And generally that's true - it's really hard work to save a thread where someone has started attacking people rather than debating the topic.
So my suspicion is that it's endemic to the format and the method of managing the various forums. Bland forums with a few "key" attracting topics gain more new members than those forums that represent a large variety of conflicting viewpoints.
That, however, is merely one small part of what makes some threads interesting and not bland. There are certainly ways to encourage interesting threads that are not controversial.
