Because, insofar as relatively large non-specialist (comparatively speaking) news aggregators go, HN is the only one where smart, well-spoken individuals still form a meaningful part of the user base. In fact, apart from a few highly specialized subreddits, this is the last major site where I still read comments at all, and I learn something new on a pleasantly frequent basis.

Also, it happens to be the last major news-related website I know of with what I would call "good web design". That helps more than I'd care to admit.