VW spelled it out last year.
VW spelled it out last year. Cost. When you build a diesel vehicle, there is a cost premium due to the higher cost of the powertrain (more robust engine, higher cost of after-treatment methods for emissions) and another premium for the hybrid powertrain (electrification). A diesel-electric hybrid has both premiums is thus prices itself too far out of the market.
Diesel costs more at the pump, but has more energy density than gasoline, which is why it offers a higher MPG than gasoline. I did the math a couple of years ago on the cost vs. benefit of a Passat TDI versus a gasoline version and it was significantly cheaper to go diesel over time. For those who primarily drive on the highway, it also beats a Volt and several other hybrids.