To get diesel clean enough for emissions concerns the crude used needs to be a low sulfur "sweet" crude and this adds to the base cost to produce it by the refiners. It is also why the US government has continually looked the other way as the Saudi Arabian government, and its elites like the bin Laden family, funded Wahhapi Islamist extremists and terrorists for decades.
Actually the price for diesel and gasoline should be double what it is today. The taxes are needed to repair the 50% of US bridges that are in danger of falling down. We are freeloading on the work done after World War II in the 1950's and screwing our kids in the process.