Weight distribution has less to do with how much the braking effort is divided front-to-rear than weight transfer does. Ask any drag racer about braking's opposite; acceleration, and he'll agree. Suspension geometry also plays a role. But SUVs, with their relatively high center of gravity and (usually) softly sprung suspension tend to transfer quite a bit more than a car would. I've not done the math specifically for a GM SUV. But on average for a sedan, the braking effort is 80/20 front vs. rear. It wouldn't surprise me if an SUV was biased even more. The incremental cost of the rear system, when compared to the incremental improvement in braking, is typically quite high. It doesn't surprise me that GM doesn't offer it, for that reason.