Since none of the six responses to this comment so far have the correct explanation, here you go: HN's ranking algorithm automatically heavily penalizes posts with high comment to upvote ratios, to stop presumed "flame wars". This system is imperfect and often removes interesting discussions, but they keep it in place anyways as it usually is flame warring that is suppressed. This system kicks into high gear and then progressively penalizes further as comment totals exceed upvote totals (or thereabouts).