One thing the HN community values is really good, well-executed products, and it's instructive to learn what killed this great one. I'm sorry to see that it was mostly antitrust shenanigans and competitors catching up (flights.google.com is very good now) that made the gap between Hipmunk and others fade.
Hipmunk however has made searching for flights better than if it had never existed at all, and I am thankful for that. I still don't think the UI of Google flights is quite as good as it TBH.
There are many reasons why this could be the case, only one of which is naked self-interest on the part of YC to promote itself somehow. For example, it could also be correlative: YC funds interesting companies; HN covers interesting companies; thus you would expect that HN covers YC companies frequently.
Hipmunk was not just "a failed startup shutting down"; it was founded by a YC alum, as you note, but it was also design-focused with a distinct UX, good at PR, taking a somewhat combative approach in an industry popular on HN, etc., etc.
Most of that 96% of business do not have even one or two of those things?
In a perfectly fair world, companies would get attention based on the value they are providing to society, not on the level of VC funding or particular affiliations.
In particular, labor rights, non-profits, legal technology, and governance are generally under-reported despite their larger potential impact on society, largely because they aren't attractive to VCs looking for out-sized returns on investment.
Companies don't have to have any relationship with YC to be linked or discussed on HN. Just... look at the front page. Plaid, for instance.
Plaid's posts were third-party articles, and they got purchased for a billion. This is ad copy. Users would not have upvoted this, because it's not on-topic, were it by a non-YC company.
I didn't even intend to be implying relevancy was required on HN (my submission history is filled with articles that aren't relevant to much of anything) with the original comment; my statement was intended to give context. The second comment, though, was devil's advocacy that I'm still going to stand by: it's nothing but silly to act like ad copy is relevant to HN inherently.