The problem is politics is so opinion and status-based that it crosses this magical threshold where even otherwise level-headed people just lose it. It's toxic beyond what even HN's regular moderation mechanisms can handle. The other thing is, it's easy for politics to drown all other topics - political issues speak well to our "someone is wrong on the Internet" instincts, and there's always something outrage-inducing to be posted.
While I personally don't mind an occasional political discussion (and I like when it's not political per se, but more e.g. economical or military-strategy type, in which people post many interesting facts and ways of reasoning I haven't seen before), I wouldn't like those topics to keep dominating the front page (happened a few times in the past).