I bet it's just a cost/benefit analysis. An API is a way to get more eyeballs by motivating 3rd party developers to integrate and publicise your service. HN does not need that: it has enough traffic as it is, and given the target audience, you would see an instant proliferation of half-assed apps hammering its endpoints. So it would be an additional cost for no real benefit.

The current situation (PG and friends optimise a basic but very accessible website, and a handful of third parties build APIs on top) is much more manageable.

My impression is that pg wants to encourage the hacker spirit by providing a bare bones service which could easily have a 'hacked' api built upon it.

i had the same question in my mind . Even reddit have there official api .