Because there's nothing worse than seeing 7,000 comments on a Reddit article and 99.7% of them being complete garbage. That's discouraging to a community. Why even bother posting when someone's lame joke is 347 votes ahead of yours. So I'm a complete asshole on HN. When I see jokes or one liners I downvote away. I feel guilty and sympathize for the poster but... I like the noise free environment too much. So I wipe away the tears and downvote away.
An equally good article/discussion is what led me here. Of the aggregation sites I know of on the web, this place is currently the only one that retains (for the most part) it's signal to noise ratio in the comments.
Reddit anymore, even though being subscribed to very specific subeddits, is just frustrating to use. It seems to me that the average age of the user has shifted downward as the site has grown. Couple that with a lack of moderation, and comments are all now one line, or a 'funny gif' response, which, by the way, are strictly outside of reddiquette...
Slick attempts at humor don't certainly get downvoted, they may not be the top comment. I still think that the reddit model when brought down to Subreddits still works. I am part of some very tiny subreddits and there is still a free flow of high quality information.
Glib and offhand attempts at humour are not much appreciated here, even though they might contain some wit, which reddit would gobble up.
The HN audience seems to upvote comments that are analytical, insightful and informative. This takes some adjustment for reddit heroes who can reap great karma with a well-placed meme.