I'll not try to speak for Rich (see Mal's ref for that), but I can speak from personal experience.
The first and easiest answer to avoiding non-firearms related threads and forums is found in the first line of our policy statement. TFL is "dedicated to the discussion and advancement of responsible firearms ownership." That's a pretty big chaw to chew right there.
But your question is a good one--and frequently asked. In the past, some members believed knowing more about each other would foster a closeness or unity among members. In my experience, the opposite is true. When we permitted some off-topic subjects to be discussed, heated arguments (involving religion, abortion, sexual topics, etc.) became more prevalent than firearms-related discussion. The subjects became more than distractions, they became divisive issues severely and adversely affecting our firearms-related goals. Therefore, though other subjects surely are valid points for discussion, we have advised taking them to other forums, to email, or other non-TFL venues.
Personally, I believe it's a shame we cannot wait until we restore our Constitution before going for each others' throats. If we don't soon rein-in our government, they will TELL us what our opinions are and these other hot topics will become irrelevant. Besides, we have more than enough arguments (and needless bitterness) concerning firearms and civil liberties. I don't think the Members or the Staff could handle broadening the scope of our various controversies.
Subjects not related to firearms and civil liberties most assuredly are interesting. If no other boards fill your needs you probably are not alone with your interests. With absolutely no bitterness or sarcasm, why don't you look into setting up your own bulletin board with forums of interest to you? It might be a bit expensive. If you don't want to fund it by yourself, find friends, mentors, advertisers or even members to help out. Be forewarned, greater than monetary expense will be the strain on your time and nerves!