Strength and reliability is the primary factor, something that a diaphragm simply cannot match in a high-boost BOV environment. Diaphragms have their place (we use them in our wastegates), as they offer low friction and good sealing capability. However, a rolling diaphragm is only designed to receive a net pressure on one side, which essentially "inflates" it against the walls of the chamber in which it resides, exactly how it is used in a wastegate