This may be partly because so many thoughtless people feel qualified to enter a political discussion (e.g., about basic income, or immigration), whereas they couldn't even pretend to understand real-time operating systems or functional programming enough to have an opinion.
Part of the beauty of HN is getting opinions about things that matter in the real world, from people who really think about things.
This is true, but I suspect the biggest reason is that politics is more prone to Crony Beliefs ( http://www.meltingasphalt.com/crony-beliefs/ ) than almost any other field of discussion.