It just comes down to dollars, and a chicken-and-egg problem of sorts that nobody here has really mentioned. We've already discussed the price of diesel fuel in the USA. There's something you're forgetting though.
Diesels are more expensive to build, more expensive to certify for emissions, and more expensive for mandated in-use testing later on down the line. Therefore manufacturers are reluctant to offer them because they have to invest a lot upfront. Consumers have less choices then, so that may be helping keep demand down. With demand uncertain, automakers are not going to be taking big risks right now. The United States has the strictest auto emissions laws in the world. Some other notes:
1. VW has huge resources and wants world domination essentially, so they can absorb the cost & risk of introducing it. BMW sells expensive cars so that helps.
2. Mazda has always been quirky brand so you can see them doing it.
3. There are a lot of emissions components, and every emissions component has to have a diagnostic monitor: The cat, the particulate filter, the urea injection, the EGR circuits, all of that. That means more potential check engine lights for the customer to deal with and more cost to the manufacturer for development, certification, and in-use emissions testing. Certifying a diesel requires special equipment due to the particulate filters.
4. Gas engines are improving quickly and are way easier for meeting emissions