Here's one reason "smart" quite smartly did not offer the diesel engine in the US.
The 40-hp", 0.8-liter turbocharged diesel engine delivers abundant noise and the slowest acceleration of any vehicle we've tested. The ForTwo took 23 seconds to accelerate from 0-60 mph, almost twice the time of other small cars. (In fact, we almost ran out of track trying to get it up to 60 mph.)"
Consumer reports reference
http://tinyurl.com/yxdypg
The 50hp diesel does 0-60 in about 19.3 seconds.