I've talked to various people in the oil industry and there is a simple answer: demand. The supply, in essence, is fixed. They can only get so much diesel fuel out of a barrel of crude, period. Gas production has increased by using a process that "cracks" oil to give a higher volume of gasoline, albeit at a lower quality. Can't be done for diesel.
So, EVERY house that gets built that uses fuel oil and EVERY new vehicle that uses diesel fuel is using a fuel that is quite finite in production. The price will continue to rise and will not go down.
Fortunately, for gasoline users, when the first of 4 new Ethanol plants goes online this Fall and makes almost unlimited Ethanol from crop waste and "garbage", there will be relief to gasoline prices.
At the truck dealership at which I work, more and more people are asking for gas engines instead of diesel. I completely sympathize.