Well diesel is a byproduct of refining crude oil. In the tower, different things come out at different levels/tempertures. Gasoline is made from the naptha product near the top of the tower, the naptha goes to another process to make the gas we use. Moving down the tower to the hotter temps, you get jet fuel/kerosene, then diesel (high sulphur), then VGO (vacumm gas oil), then ATB (atmosphere tower bottoms). The VGO and ATB's can go to another process to make more of the above. Eventually you get stuff used in asphault.
So high sulphur diesel comes out of the process without doing much to it. BUT...these days the fed says the sulphur needs to be much much lower in diesel for EPA reasons. So diesel is sent to a hydrotreater. This process uses hydrogen to pull out the sulphur. Yep there is a big pile of sulphur lying around after.
So all this extra process cost money. But I'm sure with the way things are going up...it's not the only reason.
Plenty of better and perhaps more accurate web pages to read if you google it.
Rob