The answer to your question would be an economic's answer. Drums brakes are much cheaper than disc brakes and have a rather good reliability but not well as the disc brakes of course....(Answer this then,why doensnt every car come w/ cd players if that has been around for like ever now and are cheap as hell?) Eventually the drum brakes are going to be phased out when the disc brakes parts become cheap as hell too,what a coincidence!