Specifically, says CAR (via Autoblog), the Rocketman's narrow width and short length--the concept was just 6 ft. 3 in. wide and 11 ft. 3 in. long--made stability control, now mandatory for top safety marks, very difficult. The small structure itself also likely presented problems for crash testing.
Whether it would have been offered in the U.S. was another matter, however, so the cancellation, if true, might simply mean one less piece of fruit on the forbidden tree.