DiverBuoy, look at what happens as one descend while wearing a thick wetsuit.
On the surface you have buoyancy distributed all over your body from the wetsuit. As you descend, that buoyancy is reduced as the wetsuit is compressed. To stay neutral, you replace that buoyancy by putting air into a BCD bladder. To not have any effect on trim, that air bubble in the BCD needs to have the same center of buoyancy as did the distributed buoyancy of the wetsuit that it is replacing. While the air in the wetsuit stays in place when it gets compressed, that buoyancy needs to be replaced. It gets replaced by air in the bladder or wing --- so in one sense it is the same effect as if the air in the wetsuit migrated over into the BCD bladder (It doesn't physically do this, but the net effect is the same as if it did.)
Do this simple mental experiment ------ imagine that the BCD bladder was down at your feet. Now assume that you are at 15', neutral, and properly trimmed horizontally with no air in the bladder. What happens when you descend and put some air into the bladder to compensate for wetsuit compression? Pretty clearly, you would have a floaty foot problem.
Repeat the mental experiment with the bladder up on your head --- pretty clearly you would have a floaty head/foot down problem.
Somewhere between those two extremes is the right position to minimize trim change. That's why that standard BCD or BP/wing is designed with the floatation on the upper torso.