Diesel fuel is not a byproduct of gasoline but rather one of the products that is generated from processing crude oil via distillation. You can not take 100 gallons of crude oil and convert it into 100 gallons of gasoline, instead you get a percentage of the total as gas, some as diesel, some as fuel oil, some as tars, etc.
Thank your government and the newest environmental regulations for the increasing gap between diesel and gasoline. They are doing everything they can to keep higher mileage diesels out of the consumers hands. It seems ever time an automaker figures out a way to meet the ever-tightening emmissions regulations that the government tightens them further and send the automakers back to the R&D lab.