Check out this pic, the body lines on the doors are on a different angle then the bed line, also a hair high. It drives me nuts all of the time as it makes my bed look like it isn't mounted right, especially when looking out the side mirrors down the side of the truck.
I always thought this was the reason behind not putting them in the rear pillar. Otherwise why would gm spend the money to make another part when they could just take the post off a standard cab.