Diesel is a world commodity now, unlike it was just a handful of years ago. The US has to compete for where the diesel deliveries go, with the likes of China and Europe. If you are CalTex (Singapore) and you are trying to decide where your tankers of Diesel are delivered are you going to go to Europe for $5/gallon, China for $2.50/gallon, or US for $3.50/gallon? China is a short boat ride, so you can do more shipments (quicker turnaround), US slightly longer haul, or Europe LONG haul? We have to compete on that world market. Gasoline isn't like that all over the world. Its not used on such a high level as it is in N.A. a majority of the world is on diesel.
Yes, it is much much more complicated, but thats the Cliffnotes version.
Yeah I want a new truck, but your right, its hard to justify the $8k upgrade for a diesel when the fuel is $0.50 higher per gallon at the pump. And your gas milage is marginally better most of the time, and only tremedously better towing. How much do you tow? Right? Since I bought my '02 in the fall of '01 Diesel has only been cheaper than gasoline approx. 10 months. Yeah I keep stupid notes of things like that. When I paid the at the time $6k upgrade, I did the economics and figured in the long run the diesel made infinate more sense over gasoline. little did I imagine that the price curves would end up with an inverse relationship to each other for so long.
Diesel is a simpler process to "crack" out of the petroleum, so it is cheap. less refining process, but the demand charge is what is killing us.
I dont have an answer, as I ask my self this question damn near daily.