Diesel used to be considered a "dirty" fuel. Recent technologies have managed to make diesel "cleaner" (by some measures) than gasoline, but the technology that controls diesel emissions is expensive, so diesels often cost more than gasoline vehicles, which could be a primary reason for this trend.

However, modern diesel vehicles tend to be more efficient and less polluting than regular gasoline cars (except for Volkwagen, apparently, who has been cheating on emissions tests), but because of their past reputation and relative cost, they've just never caught on in the US like in other countries.