Due to after treatment(with some catalysts having containing enough precious metals to purchase most small countries),EGR, OBD, and High Pressure Fuel Systems, the price of the "simple" engine is now way beyond what gasoline engines cost. Technology is expensive and not always the answer.
Sad but true... But one most remember how we (industry and EPA) got to this point. Some manufactures (I wont mention any names) brought the tightened emissions regs on themselves by doing things against the spirit of the regulations..i.e. CHEATING.