Because time and time again it diverts into a polarizing, political, religious, or similar discussion which is specifically forbidden. As mods, we can't catch everything without users' help. In fact, sometimes we post in threads that are ultimately locked. But when things are specifically discussed in the rules, or are of topics well understood to create polarity, argument, etc., they can and will be nipped in the bud.
It would be great if magically the board could be run to everyones' liking all the time. Alas, it is run by people who do not operate boards as their day job, and do the best we can. The Notify button is a great tool for folks to flag things or get their say.