I think a big part of the problem is that maintaining an expert community is expensive. In addition to hosting costs, moderation in particular takes a lot of ti

I think a big part of the problem is that maintaining an expert community is expensive. In addition to hosting costs, moderation in particular takes a lot of time and effort to ensure quality of contributions remains high. Eventually, the people who host such a community need to get paid one way or the other, otherwise they simply won't be able to keep it up. Such communities will eventually die due to poor moderation (because moderators aren't compensated, so don't bother moderating at all or are driven by ulterior motives such as ego) or due to attempts to monetise the platform which kill the user experience. In that context HN is different in that it has a lot of resources dedicated to it, including a very diligent moderator, without any apparent need for the site to break even or turn a profit. I'm not sure how YC views HN in the context of their business model, but it seems that HN is effectively subsidised by its broader business which IMO is why it has managed to thrive for so long.