To stop the place becoming like reddit, where all insightful comments are buried under a torrent of jokes. I've had my fingers burnt a few times and then sulked for a few days after one of my many hilarious quips was down-voted into oblivion.
My comment stays at 12 upvotes. Certainly nobody would want jokes to surpass serious comments, but this is not achieved by killing every single joke. Also, there's another joke on that thread.
Sth. that one needs to get used to on HN I think... Randomly picking on arguably off-topic and arguably offending posts proactively. Also, I'm shocked seeing that the mod here has so low karma and so short history of HN usage.
The system is far from perfect, but I've never got the impression that the rules are applied malevolently. I wouldn't like the job of moderating comments, it's really hard and ironically people tend to have a zero tolerance policy if they feel hard done to by an error in judgement on behalf of the mods.