I think it's partly because there's still a rather strong ideal of what good HN content should be like, backed by active participation by reasonable moderators

I think it's partly because there's still a rather strong ideal of what good HN content should be like, backed by active participation by reasonable moderators who enforce the guidelines when they're blatantly violated. There's also a text by pg where he explains some of the design (or "anti-design") choices that shape the community in a desired way.