A) There are many external factors that can implicitly cap the number of upvotes/comments. (time submitted, amount of competition, etc.) HN has repost rules to alleviate this problem: would Apply HN posts be able to repost too?
B) Not to mention that it encourages sockpuppet voting/commenting, especially since there is a high reward for doing so.
Product Hunt, for example, thrives on "how can I get exposure for my startup submission outside of the intrinsic quality of the startup itself?" and it would be an improper fit for HN.