Because of the associations to pre-existing intense controversy. Those act like magnets pulling discussion in their direction, away from the topic at hand.

On HN, we're trying to open up a space to discuss new things, unexpected things, curious things. Those exert a weaker force on attention than the sensationalized hot topics that already exist in people's minds. When comments hop from a new topic to an intense pre-existing topic, that creates a path to flamewar hell. It's moderation's job to restrain and undo that, so that the more curious, weaker topic can get attention again. Does that answer your question?