Diesel cars are very clean these days. The problem is that until recently in the US the standard of diesel typically found was of a lower quality than EuroDiesel. Now it has tighter regulations, but diesel cars still carry the stigma.

Not only that, but diesel is rare in North America. You cannot guarantee that the next gas station you pass will serve diesel. That alone is enough to put people off buying them.

Ford, General Motors and all the Japanese brands offer diesel models of their fleet in Europe but they don't tend to sell them here.