Why not use an RNP of 0.15-which all modern Boeings can do (not sure about the AB-ours can't use less than .30)-with the reduction in minima such an approach could accord?
Because you have to take your crews off line and put them in the simulator and that costs money. Because the regulator has to take a long hard look at your books and the training you plan to provide. Because you have to have the resources to develop the training material and many airlines are resource poor as they try to emulate safety on the smell of an oily rag.