The problem is that the voters for these awards are too concerned with flashy numbers, like blocks or steals per game. Although getting a lot of blocks or steals CAN be a good sign of good defense, it doesn't mean it always IS.
I mean, it is one thing if you get 3 blocks per game, and another if you get 3 blocks and 5 fouls per game. Look at players like Stiemsma, last year, who was good at blocking shots, but overall was not a great defensive force. Fab Melo is another example of this, though he is still raw and developing.
So the most deserving player for DPOY will probably never win as long as Ibaka is grabbing headlines with his shot-blocking, even if the rest of his defense is still pretty mediocre.
At the moment, the players who deserve it most are KG (for the first 1/3rd of this season, we were the league's best defense with him on the floor, and the league's worst with him off of it; that ALONE should, in a just world, make him automatic DPOY), Tim Duncan, and Tyson Chandler. Honorable mentions to Roy Hibbert, Tony Allen, and Avery Bradley.