Its very simple. Refineries can only refine so much oil into gas/diesel. When the prices of oil started skyrocketing and prices at the pump started going up, the oil companies running the refineries decided to produce more regular gas than diesel, because they believed the big companies that run trucks could absorb the higher cost easier, thus less complaining.
Which would you rather have complain, the regular consumer numbering alot, or the bigger companies numbering less. So the decision was to make more regular gas, less diesel. Yep, Diesel went from being cheaper than unleaded to alot more because refineries decided.. lets stick to making the mass of common folk happy and hope they don't complain too much.