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?

Yes, thanks for clarifying.