Look at the reasons why sales are lower. Diesel that cost more than gasoline even though it is much cheaper to produce the taxes are higher on diesel than gasoline, the EPA and Carb for a start. It is a shame more Americans have not experienced modern diesels they really are nice to drive and easy to keep.
My brother has a 2013 BMW X5 35d 6cyl turbo diesel he bought it new and it has 150K on it, he has spend $450 in actual repairs plus regular maintenance of oil and fluid changes and filters, the only repairs were glow plugs with the module and a serpentine belt.
It gets 30+ MPG which is great for a 5,000lb all wheel drive vehicle.