The comment is at top because it has constructive criticism, which is quite a bit different than just a "negative" comment. Constructive criticism can just as well be delivered with a positive tone and I've seen those rise to the top as well. Likewise, there are plenty of negative comments that sink below if they're devoid of useful, entertaining or otherwise noteworthy content.

> "Positive" comments only serve to make people feel good

Even if this were true, positive feelings are a powerful motivator.

But it's only true of vaguely kind/affirmative comments, and not all positive feedback can be categorized that way -- it's less common than it should be, but I've observed constructive positive feedback where the purpose is to describe specific positive effects and mechanics.

> "Negative" comments, on the other hand, often make flaws and problems more obvious.

Depends on what you mean by negative.

If it's really pretty much about statements of fact making explicit the negative connection between some attribute / process and a goal under discussion, then sure.

But just as positive comments can be vaguely affirmative and only serve to make people feel good, negative comments can be vague and only serve to make people feel bad rather than illuminating a problem.

You could argue that making people feel bad is a powerful motivator, too, I suppose, and I suspect that if you do feel that's an important tool, people would appreciate knowing that up front.