Your thumbs can only stretch so far. See, on the computer—where everyone first learned how to design interfaces—it didn’t matter if the buttons went at the bottom or at the top because the mouse could go anywhere. In fact, the top made a sort of sense, because you could use drop-downs and all kinds of other nice things like that.
On the phone, though, your thumbs like to click things at the bottom of the screen. Not to mention, reaching across to the top might cover the screen with your hand—that’s no good.