Most people (me included) are not really in the target audience for YC. I have no interest in starting a company, and even if I did I wouldn't want to do so in North America.

You also see some posts on here about some YC founded company or other with open positions, which is a wider audience (so that helps the equation I guess).

My guess is that these two target audiences together is enough for them. It is not like HN is a heavy site, nor does it change much over the years. So with smart coding (i.e. a compiled language) and hosting my guess is that moderation time is the bulk of the actual operating costs.