Why Isn't This in Production?

A couple shade tree mechanics in southern Maryland have cobbled together a diesel Honda Insight. With the electric drive only partially working they are getting 78 MPG driving the the Pacific Northwest. They expect to break 100 MPG once they work out the bugs in the electric portions of the drive train. Admittedly modern diesels cost more than gasoline engines but they run cleaner and last longer than those same engines while getting better mileage. Why isn't Honda or Toyota outfitting the Insight or Prius with a diesel option? The target markets have already shown they will pay the extra coin.