Simple - cost. If a hybrid costs about $3,000 extra, and a diesel costs about $1,500 extra, then a diesel hybrid could cost $4,500 extra. Say you take a Prius at 50 mpg and add diesel to get 75 mpg. Over 100,000 miles you use 2000 gallons with the gasser, 1333 gallons with a turbo diesel, a savings of 667 gallons, $1,333 dollars at $2/gallon. No payback after 100k miles.