What is up with the cost of regular gasoline compared to diesel?

Ok, this is more of an open ended question as opposed to a informative blurb, and I hope that I am not deluged with conspiracy theories, but why is diesel so much higher than gasoline?  If I remember my (barely passed) chemistry classes, diesel requires less refining, so I don't think that is it. 

Any ideas? Any ideas backed up with some facts?