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.
The Rocketman had been thought to be roaring toward a 2014 production date, with public reception and company officials in favor of the sporty small hatch, even as recently as December 2011.
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.