I would agree that diesel has the highest demand of all the fuels. But why the higher cost due to environmental restrictions? They've moved to ultra low sulphur fuel at all stations selling diesel and cleaner exhaust emmisions on all vehicles sold. Even stationary engines have move to Tier III or Tier IV standards.
Sounds like BS to me.
It looks like it's a combination of things, and with all of us being a captive audience; they can use the slightest excuse to raise prices.
This link goes into more detail:
http://www.ehow.com/about_5472995_diesel-fuel-cost-gas.html
A few points from the article:
1. The combined taxes for diesel is nearly 7 cents higher per gallon than for a gallon of gas.
2. While refiners were making the EPA mandated sulfur modifications, diesel production was temporarily shut down resulting in shortages, which also pushed prices higher.
3. They're recouping the expenses for the sulfur mods too.
4. The global demand for diesel is high but the amount of diesel which can be refined from a 44 gallon barrel of crude is less than 1/5 of its contents.
5. As refiners produce more home heating oil, the amount of diesel produced drops by a corresponding amount.