That's true in general, but the business of tech can sometimes benefit from local knowledge. A submission about the best lawyers for tech startups in Singapore wouldn't really be appropriate for HN since it wouldn't benefit a large part of the audience.

It also helps to have a site where there's a high chance that the person you're talking to is close enough to meet up for coffee. =)

This is a fundamental flaw in communities on the net in general, they don't gracefully combine different scopes.