I personally dislike the Smart cars, but they do actually have decent crash results from a variety of tests (NCAP and IIHS). I think they would have been a huge success in America had they been priced more fairly and gotten at least 100% better gas mileage. If you look up the fuel economy numbers, it is a bit embarrassing... Yet, I still file a Smart car buyer as the same type of person that purchases a Prius.
Canadians apparently go ape$&#@ for them and I guess it does not help that they were only sold in the US as a grey-market car up to the 2008 model year.
As for the diesel powered Smart and Mercedes-Benz S-class cars not making it here, that is probably due to the fact that many of those diesel engines have had a hard time passing North American emission standards. (Particularly those of California)