Another reason is fuel price.. When gas was close to $2 U.S. per gallon, diesel and propane were great alternatives. Now, at least around here... Gas prices for the cheap stuff is between $1.03 to $1.20 (that is between Tucson, Arizona and Las Vegas, Nv).
The average Diesel Price I saw was at least $1.49/gallon. Propane, at least around here, is between $1.29 and $2.00/gallon depending on where you get it.. (it is $1.50/gallon delivered to the house).
Also, on most diesel motors it does cost more for an oil change.. more oil, and last time I looked, the filters cost more (at least for that old non turbo Ford Diesel that was in my parents 1980's f-250).
But, you can get more power out of some diesel motors than you can from the same size Gas motor so then the mileage is better and it would be cost effective.. (look at Rick Horowitz's Truck when pulling his X.. almost the same mileage as the X gets by itself)..
So it all comes down to the actual operating cost of the vehicle, plus it usually costs more for a Diesel Motor.. If you pay $2k more for a Diesel Motor, how much money are you really going to save from fuel savings over the life of the vehicle after you pay for the extra cost of maintaing a Diesel motor?
To me, it doesn't seem worth it yet... unless gas prices climb, and stay there.
~Mark