It would be difficult to do properly, unless it just cached only what you were looking at.

Most links in HN are to external sites, so you wouldn't want to be caching them on your phone. Then there's the amount of traffic to HN meaning that you would be caching massive amounts of data only to get mostly unusable links, and some comments about them.

I agree that it would be nice to keep reading things after you go offline, but I think the technical side of it would be prohibitive.