a) People are interested in YC companies and
b) YC has the connections to get the posts there... If you had the connections, would you not use them?
However as already mentioned by other users, it is normal for people that are closer to YC, to cause a viral effect for people they know.
I suppose that what we have to consider(as general users of this site), is if we get valuable information from this site. I have been reading HN for about 2 years and I believe, it has been a good source of information. As all sources of information you have to consider the articles that are more relevant to you. Being "top" ranked in HN will not make your startup/application better if it doesn't benefit the user.
It's a shame that smaller startups don't get as much coverage on Techcrunch anymore, but hopefully others will step in to fill the gaps. TheStartupFoundry looks promising though!