I think that the true reason is that people just generally don't know how to work on and diagnose diesels as well as they do gasoline cars, diesel can be harder to find when you are getting low and need fuel ASAP, diesels are usually more expensive to buy and modify and the good/efficient diesel cars are VWs which have an unjust stigma of unreliability around them because VW owners tend to not do the maintenance that they are supposed to and just let them fall apart.
I'm in the middle of an eco-orriented build here:
http://ecomodder.com/forum/showthrea...mpg-31997.html
I haven't found too many others building from the ground-up specifically for fuel mileage, only the occasional question of which tires, nozzles, tunes, etc.. would be best for MPG without sacrificing power, traction, etc..