Well, for starters, old.reddit.com on mobile safari doesn't put an overlay telling you to use the app instead when you get a deep link.

Also it loads much faster. Imagine if HN were re-done with a lot more glitz and current HN was turned into old.news.ycombinator.com? I bet a lot of HNers would prefer the old interface especially if there were some annoying UI patterns in the new one.