The problem, according to Automotive News, is that the individual lighting elements are too small to meet current US safety regulations. The rule states that each lamp should be 22 square centimeters (8.6 square inches) and the whole lighted combined area must be 50 square centimeters (19.6 square inches). In the individual sequence, Audi's lights are much smaller than that. As for the Ford Mustang, which has had sequential turn signals since 2010, this system is legal since the first lens is large enough to meet the required size, so the other two lights really aren't even necessary.
It sounds like it could be some time before we see these turn signals on Audis in the US, which is a shame because in addition to their styling bebefuts, we think they're more effective at signaling the vehicle's intended direction of travel, and they do a better job of grabbing the eye. Scroll down to watch a pair of videos showing the headlights in action on the redesigned A8/S8 as well as the recently updated R8.