I have never run a car company...
It may seem short sighted to us (myself included), but ask the following questions.
1) How much have I spent on R&D to date? How much more do I expect to spend on R&D? R&D includes the cost of designing the engine, getting EPA approval, developing the tools and methods to mass produce it, and training the mechanics/technicians who will repair it (and surely, things I haven't thought of).
2) Am I willing to spend $250 million in plant upgrades to begin production of this engine?
3) Is the truck buying public willing to pay up to an extra $8,000 to buy my engine? How many miles to they have to drive to break even for that extra $8,000 cost?
4) What is my profit margin per engine at $8,000 each, and how many do I have to sell to break even for the R&D and plant upgrades?
Given the average consumer's short memory, and the recent slump in new vehicle purchases, I can't say I blame any of the manufacturers for halting development of their small diesel engine. I wish they would press on, but I don't see the same picture they do.