The reason that BMW kept the same design lies in the contemporary magazine articles of the era. Compared to other bikes of the time, the ATE caliper setup was regarded as "different" but very capable and highly regarded. ATE had considerable experience in automotive brakes, and the design principle was sound. The single disc had its limitations, but the double disc didn't. The first year criticism of wet weather braking performance was largely overcome with the drilled discs fitted for the 1975, so with the 40mm calipers fitted for 1976 onwards the double discs were considered among the best brakes then available on a production bike, with impressive stopping distances "in the day". Consider that this was a "pioneering" phase for disc brake technology on bikes, and remember the nasty cable operated calipers on some cheaper bikes and the fully enclosed setup on early '80s Hondas and it becomes quickly apparent that the technology was still finding its feet.