Remember that the now-defunct smart forfour was essentially a Mitsubishi Colt (4-cylinder) with a tridion in the middle. I'm not sure if it even met the EU4 pollution standard (but maybe), and now EU5 is on the close horizon. The US standard is much stricter than EU4 and very close to EU5. The two showstoppers for diesels in the US are the ability to physically carry the significant required equipment (right now limited to aqueous urea injection), and the infrastructure to re-fill the A/U tanks (per EPA, a dealer-only operation). For a smart, that is 1/2 gallon from somewhere per each 10 gallon fill-up (1:20).