It's all about reputation. Not that Coach K isn't a good coach, and not that he doesn't have good players, but picking a preseason number 1 is all about hype. Duke generates some of the best hype in the country, thanks to a bunch of ACC-biased commentators. Also, there can only be one winner, and it's not always the best team that wins. Sometimes, it's just the one who doesn't hit a cold streak in March.
in the last 8 years duke has been chosen #1 at least 5 times and didn't win the year they won ariz. was #1