It's an interesting question. Primarily, I think it's because HN doesn't allow you to downvote instantly or even after a lengthy period of time. I think it's tied to total karma, but someone would have to provide more information there. Regardless, that single change probably makes a big difference.
Compared to Reddit, I've had some comments go into the tens of negative karma points within five minutes of posting. It wasn't because it was low quality, but because it wasn't the "correct" view to have in whatever subreddit I was engaging in. The downvoting there is practically militant.
However, as someone who usually holds a minority view on HN, I don't think the system here is perfect either. Usually an echo chamber forms because the dissidents don't last long and leave. If you reward the ones that stay the longest with downvote capabilities, it would explain my general experience quite well. But again, it's nothing compared to Reddit.
Note: I recognize this is a conversation on karma, which has a rule associated with it, but I hope we can make an exception here given it's a good faith discussion between Reddit/HN :)