It's my opinion (which means little
) that the low roll center on carpet it utilized to reduce suspension jacking and hence traction rolling, eventhough I think a lower roll center creates more overall traction. The increase in traction can then be adjusted out via springs.
With a higher roll center the lateral forces of the tire pushing back on the car have more of an "upward" angle, which causes the suspension to "jack" or lift as the car corners.
I remember David Spashett mentioning that with a low roll center the car has a tendancy to "roll" the outside of the chassis into the ground during cornering. With a high roll center the car has more of a tendancy to try and pick up the inside edge of the chassis which eventually causes a traction roll, which is always a problem on carpet.