I think it's due to the fact that many HN people prefer to give honest analysis/opinion, even if the truth hurts. In the long run, it's probably more useful that way to whomever posted, to get "real-life" feedback at least from a technical perspective.

The question did make me think, whether there's a small part that supports the stereotype(?) of "smart" people, or those working in IT/tech, having less empathy and emotional consideration. Kindness can be seen as unnecessary or even harmful sugar-coating..