I had a VW diesel rabbit in the 70's that got 40/55 all the time. If VW had put a 5 speed transmission in it the milege would have probably been 50/65.
It is about the money and because Chrysler abandoned the Turbine car for the same reasons. Only one moving part, one spark plug, little iol usuage and high mileage.
So far, Hillary is the only one driving a green vehicle.