I fear that trolling and the circulatory nature of both subjects and arguments (Will puts it most succinctly) might be the cause - that and the ever present boredom (at least to me) of being drawn into some sort of Fascist/Communist debate whilst entering what is a superficially innocent thread.
Having said that, surely it is not beyond the wit of man/woman to come up with some original topics or even some radical slants on the tired old chestnuts.
Another forum (covering a totally different interest that I follow) is suffering from the same malady. Interestingly the average age of the contributors and partakers of this eccentric hobby is similar to that of the attendees of the average folk club. Facebook seems to have taken over for the younger members but doesn't seem to offer quite the same in-depth discussion and can quite quickly erupt into fiery confrontations...mind you we appear to do that quite well ourselves.