There are plenty of larger diesel vehicles that have no problems at all meeting both U.S. and European rules, and indeed they came out 10 years ago (the U.S. "Tier 2" rules came into effect for 2007 model year vehicles).

The problem is that, on smaller vehicles, it is tough to find room for the emission control equipment and it hurts fuel economy.  It isn't at all impossible to do, but reducing NOx emissions down to a proper level will cause higher fuel consumption, and consumers buy based on fuel consumption, not air pollution emissions.  Many probably mistakenly believe that their diesel vehicles were somehow "environmentally friendly" due to the lower fuel consumption, even though they were polluting the air at levels vastly in excess of a comparable gas powered car.