He was the cornerstone of the leagues best defense for multiple years at a time, when there was no other outstanding defender, that clearly deserved the award over him. Ron Artest could've stolen one or two of his awards, but we all know what did not allow him to do so.
I feel he is not on Dikembe/Mourning lvl of the nineties, but he is close. His help defense is equally good. He defends the rim like a true center. At 1on1 D, he is limited by his size, but there is nobody to wrestle the award away from him.