I don't really like the Coach of the Year award. It's never given to the best coach, but rather the coach who surprises voters the most. In essense, it's a way for the media and voters to avoid blaming themselves for totally missing on a team; they can instead praise the coach as the difference, not their own predictions.
Benson, and the shift to a running offense was a big part of their success. They had two young corners step up, and the defense was great under Mike Zimmer. Those who say Lewis was a great DC also have to admit the Bengals D has been wildly inconsistent under him, and just because you're a good coordinator doesn't make you a good coach in that area (see Brian Billick, RAC).
Good for Lewis though. He wasn't the best coach this year, but this award isn't about the best coach.