Some were skeptical when it was first announced, but I wasn't. The idea of an Assassin's Creed set in the United States during the Revolutionary War still gives me goosebumps. …there they go.
It's actually surprising how few of the games we play are actually set in the US. If they are, it's often difficult to tell; most times, it's just a fantasy world. But to have a vibrant, realistic, meticulously designed environment (just look at past AC entries) that is supposed to recreate part of our country during a very important period should be undeniably appealing to Americans. It seems like common sense.
Now, it's certainly true that Assassin's Creed III is the first "true" installment since ACII and the beginning of Ezio Auditore's story, which continued through a couple more entries ( Brotherhood and Revelations ). It's also true that all titles in question have been immensely popular, so it's not like this series is finally tasting popularity. There's also the fact that Ubisoft is issuing some major changes and upgrades for this new adventure, which is bound to make everyone stand up and applaud. Well, they'll applaud the concept, at the very least; the implementation and execution quality will be determined at a later date.
But might the setting have a little to do with the fact that ACIII pre-orders are jumping through the roof ? Of course, this will be a global product but remember, those numbers are US-only, so maybe it shouldn't be surprising to see such a reaction. Even those who may have been suspicious of the idea at first are starting to warm to it, and there's no doubt that people are excited. I really think some of that excitement stems from the environment, which we've never really seen before. The closest would probably be the Western frontier in Red Dead Redemption .
I think there's something about playing through a piece of history that's relevant to one's own country, and that's going to continue to fuel interest in the US. Just a theory.