Ford are very concerned with ride quality. lower profile tires make for a harsher ride. while enthusiasts are willing, and most desire, this change, enthusiast are not the only ones buying the cars. women and older folks still make up a significant segment of the market for mustangs. If factory cars have too harsh of a ride for even some of the customers Ford won't do it. They do not want the risk of the bad PR and poor csat ratings that could hurt sales. This is why for years Saleen and Roush and the like were always able to have better suspensions and wheel/tire combos - because the ride that results was the expectation from them; but not the expectation for the factory offering from Ford.