thepmp could be right, I’m not sure, but even if he’s not, most of the braking effort is done by the front brakes. If you’re Ford developing a car to a set budget, you get most performance for your money by spending on the front brakes.
Would cost a whole lot more to incoperate parkbrake mechanism. Would require inboard shoes or 2nd caliper as per supercars. As they are there fine. Would just require better pads and fluid for frequent track use.