It's coming to the UK "soon" according to @daily.
Content seems to be pretty heavily US-focused, and as it seems to be (at least in large part) a gossip rag, it was decided against shipping US gossip to rest-of-world customers.
They don’t have to market it, why not just make it available everywhere? I’m not in principle opposed to the App Store but this is one of those stupid things about it. The cool thing about the web is that by default, everything is available everywhere and it is hard and not foolproof to make something not available everywhere. I can read nytimes.com in Germany despite clearly not being part of the New York Time’s target group.
What reason could there possibly be for not making The Daily available everywhere? Sure, hardly anyone will buy it outside of the US but at least Apple is footing all the bandwidth costs and making it available everywhere must be a matter of clicking some checkbox. Wired and Popular Science are, for example, both available in the German App Store. (Not that I would ever want to buy The Daily but this is just so stupid.)
Although they surely won't roll out special versions to each national app store, so you do have a point there…
I'm not sure why they did it, but it's their prerogative.
Ferner, I don't think you should forget that there is still a lot of web content which is restricted by country — YouTube being a much-despised example of that, although they have obvious reasons there; BBC iPlayer being another example with similar (but far more justifiable) reasons. From those examples, I think one can guess that one of the issues may be copyright: It's easier for them to handle the copyright of just the US.
So yeah, it's harder to distribute US-only than to the world. Not sure what The Daily's reasoning is there.