By allowing only the following forms of advertising: YC startups looking for employees, companies looking for employees. Show HN threads are meant for "hey look I made this, I'd like feedback / this may be useful to some". Beyond that, everything that looks like an attempt to advertise (as opposed to honest suggestions) gets downvoted and/or banned directly.

HN has indirect benefits to YC, so it gets hosted and maintained. In that model, ensuring authenticity and freedom from advertising cancer works for the benefit of both the community and YC.

> ... everything that looks like an attempt to advertise (as opposed to honest suggestions) gets downvoted and/or banned directly.

That is so easy to game for a startup - Post something related, get friends/ pay for upvotes, repeat. What applies to quora's fall also applies to HN. How could HN (or any other niche community for that matter) survive that downward spiral?

HN tries to actively counter that - including by employing algorithmic voting ring detectors, and trigger-happy community willing to downvote and flag anything that smells of astroturfing :).

But I guess a big part of the reason is that HN generally doesn't want to have this kind of content here, whereas people running Quora embraced and encouraged thinly-veiled ads.