Because some people, otherwise valuable and great to have around, will disapprove of someone as soon as they know he/she is interested in politics. They can't help seeing that as a mark of bad character.
I don't know of anyone who thinks "I can't respect her; she has this thing about AI."
There is a tendency to downvote articles and commentaries with a political subtext with a remark on how politics is the mind-killer. I completely understand that nobody wants his mind to be killed, however, I disagree on the employed methods. I don't think anybody can really afford to ignore politics. It's a fact about any group of even a handful of people. Thus instead of shunning politics I think it's better to build one's rational defenses. Understanding that politics is a problem is only the first step. If you stop there, there will always be a big part of life where you are not rational. Therefore I suggest that, as long as it doesn't get out of hands, there should always be room for political discussions if not on the main site at least in the discussion section.