Image, too. You've got to give off the appearance that you're intense, even menacing.
No way Bowen (who's defense was obviously aided by Duncan's mere presence behind him) not winning one tops Duncan not winning one. In fact, I'd have a hard time ever giving a perimeter player the award because no matter how good they are, they can't match the impact of an elite big.
Duncan not having at least a few is the biggest awards oversight I can think of, in any sport. People can knock the most common defensive metrics all they want (and for sure, they're flawed), but the fact that he's at or near the top of all of them and has either anchored or co-anchored a bunch of the greatest single season defenses in league history and hasn't won one is laughable.