Mostly because you do not have possi in any shape, manner or form /ubbthreads/images/graemlins/crazy.gif. You have limited slip and what you are describing is how it works (or doesn't work if you prefer). Both wheels on a slick surface-both will spin. 2 wheels on a good traction surface-the one with the least traction will spin and the other will de-clutch and try not to spin to move the truck. 1 wheel on a slick surface and one on a traction surface-the one without traction will spin while the other will slip the clutches and try to move the truck.
The concept is a good one but doesn't work very well most of the time. Nature of the beast /ubbthreads/images/graemlins/frown.gif.