I don't think you can just say diesel is less popular in the US without bringing up the emissions scandals. It genuinely seems to me like companies can't deliver clean emissions and efficiency gains at the same time for it.

The scandals don't matter. The number of people in the USA who buy diesel passenger cars rounds off to "nobody." There's just no point in even bothering. Supposing you could make an ultra-low emission diesel (without cheating), you'd still sell almost none.