I think because, like religion it is an area where peoples views are so entrenched that a civil debate is almost impossible to have. Any thread on these subjects I have seen has always become full of personal insults and short on reasoned debate.
I am a member of quite a few forums, all of which except this one will discuss politics. I can argue pretty well, but I don't insult. Every forum except this one seems to manage politics discussions well, with no personal insults.
Maybe this says something about BCUK posters. In my experience, alot of them tend to be slightly isolated and less sociable than average. Perhaps also they use computers less than the average person, so are less familiar with etiquette.
And I have had a lot of people trying to lure me into slanging matches. I can control myself. A lot of people seem to type offensive comments, followed by , as if this makes it acceptable. The tone of your comment does not transmit well over the internet.
I would say don't close this thread. Then it's just another victory for the people who post with the intention of getting threads closed. I say ban the members who start insulting others. I would also say that you should try allowing a thread on politics, is it explicitily banned in the rules anywhere? If not, do the mods mind if I create one? I hope people have enough sense to debate sensibly.