I suspect it's an effort to keep the signal to noise ratio high. In reference to April Fool's gags on HN, 'dang commented a while back:
> "A word about humorlessness, since it's bound to come up. People who complain about HN's humorlessness have a point, but not because we're against humor. We like laughing as much as anyone. It's because empirically, a culture of humor means a flood of lame humor, and HN's goal is to optimize for signal/noise ratio. As with any optimization, there are inevitably tradeoffs."
> "A word about humorlessness, since it's bound to come up. People who complain about HN's humorlessness have a point, but not because we're against humor. We like laughing as much as anyone. It's because empirically, a culture of humor means a flood of lame humor, and HN's goal is to optimize for signal/noise ratio. As with any optimization, there are inevitably tradeoffs."
https://news.ycombinator.com/item?id=7506793