As a refugee from Detroit, I will answer this succinctly.
The reasons are much simpler than most would like to believe. It is call The Market Place.
Most DP are built on chassis that are designed and marketed to bus builders and operators. (This is why the door is where it is.)
They want diesel.
Two reasons: Horsepower and Longevity.
To do the engineering and testing for an engine that has a limited market will cost the manufacturer too much to recover. And, now with no large truck V-8s in production, it makes even less sense.
Matt