I consider myself somewhat new to HN, but here's what I think about it. I'm likely to be wrong, but it may be at least partly the reason.
First, it seems very useless to say “This is cool, I like it”. We know these types of comments, which often remain around the bottom.
Much easier is to write a useful objection that would grow into a thread and is likely to add to the discussion. You can point out a factual mistake, doubt something—you just need to stay more or less on topic.
The problem is often, though, in how the objection is phrased. One can say something like: “This is very good work, but [objection phrased as a question]. But still, [x] is a nice idea.” I always try to object like that. However, it takes time. If the person appropriate for the feedback is unlikely to read HN, politeness doesn't seem to be worth it. We also need to account for cultural differences. Then, many of more technical people here seem to be not very sensitive to politeness level, but very sensitive to factual mistakes.
So we end up with comments that look frank, but are useful and start discussions.
New people probably want to comment in the same spirit to attract attention and build up karma, but they may assume that frankness is the key. (What for one is a plain simple comment, for another may look like a blunt statement.)
So we end up with comments that often look frank and aren't very useful.