Why is so hard to build a similar community somewhere else? (I don't plan to but...) Suppose I wanted to build a community like HN, how do I proceed?

It's the old chicken-and-egg problem: you build a community by attracting new users, but new users will join only if there's already a good community. It's hard to get a community started.

Strong moderation with clearly defined goals and rules could go a long way, granted you had the resources to back it up long enough for it to become self-sufficient. Stack Exchange has some categories with strong moderation and rules. The problem with SE, in my opinion, is that posting there feels literally like working without being paid. There's almost no fun in posting answers on SE, while there is still a bit of fun posting on HN.

What do you want in a community that doesn't already exist somewhere?