[krapp]

Because, for some stupid reason, having your startup or blog post appear on the Hacker News frontpage is a Really Big Deal, and the staff don't want anyone peeking at their algorithms to find ways to game the system. Which means the really interesting problems they have to solve like voting ring detection, spam detection, etc. are the ones they don't want anyone to see their solutions to.

Also (personal theory of mine) maybe the code doesn't have the degree of abstraction or separation of concerns necessary to allow open sourcing without also exposing YC business concerns or parts they want to keep secret.