I bet it has a lot to do with our governments regulations changing all the time. I bet GM-Ford-Cummins have them built (on paper) but don't want to invest further with the regs changing all the time. I'm pretty sure GM had a plant near Buffalo ready to go on their 4.5L D but pulled the plug on it the last minute, probably again because of government regulations in the marketplace(s) problems. Plus diesel fuel has gone up and down a lot too. From my standpoint, I blame the governement, as usual.