I've always heard it has to do with localization. It's easy to release it in Japan first because they have it already made in Japanese. Then they start translating it into other languages, like English. It looks like Europe gets it on the 19th, and we get it on the 26th. That week difference may just be fixing the English to not be United Kingdom-style English.
Actually, now that you mention it, I definitely think that this is true. After
Dynasty Warriors 7 released, its expansions were only released via digital marketing for download. The expansions were also released in Japanese, so the translations were never put into practice. (Then Techmo-Koei decides to not give them to the Xbox crowd.
)
Warriors Orochi 3 is another example. While the
Hyper edition released in North America on disk/disc, the game was still released in Japanese. The latest entry in
Dynasty Warriors returned to the localization process, though;
however, the Xbox owners still yearn for some expansions!
I think Keith pretty much nailed it on the head; however this begs the question, how come this wasn't the same with the likes of A Link Between Worlds, Skyward Sword even Twilight Princess? All of these titles actually had later release dates for Japan. I guess there is a reason and I haven't looked too much into it, so anyone got any possible explanations?
I cannot formulate any concrete answer, since I don't have a 3DS and hence did not play ALBW, but I would assume that since the game may/may not have any voice acting, the localization process perhaps was finished early on. I guess that Nintendo might have wanted to reach their audiences elsewhere first. I think there are numerous possibilities, and heck, I could be wrong here!