I dunno. Maybe it is because I am American but when comparing all the non-america covers to the American ones, I have the distinct feeling almost every time that I'd be more likely to pick up and look over the American cover than the others. I agree the others tend to have a better artistic sense but marketing isn't about art, it's about grabbing the eye enough to get you to look at the back, like Yahtzee listed. Actually the majority of the American box art covers meet the first two criteria if not the third (viewable from a distance).
I think this is also a plausible theory.LadyRhian said:Because America is all about identifying with the main character- becoming the main character, so that's what we want to see on our box art. Admittedly, not always, but mostly. And we can't identify with a character (or characters) if we don't know what they look like.