That explains some of it, but I think it's even a narrower focus than that. I participate in several communities that are broadly technology/programming/science, and they're quite different from HN (and from each other). For one thing, my impression is that there are relatively few scientists and engineers on HN. I occasionally will see a post by a biologist or a civil engineer, but it's not typical. Even among computer people, it seems like a particular social subset, perhaps due to being hosted by Y Combinator.