Why? Simple. In June 2006, the U.S. Environmental Protection Agency required 80% of highway diesel fuel sold in the U.S. to meet new ultra-low sulfur standards similar to those needed for low-polluting gasoline. By 2010, all highway diesel fuel sold in the U.S. must meet these new standards.
For better or worse, the EPA did not require oil refiners to invest the estimated $6 to $9 billion in technical improvements needed to meet the low-sulfur standards. As a result, U.S. refiners have continued to produce large amounts of high-sulfur diesel. Because they can't sell it domestically, they've exported it. In other words, the U.S. exports diesel. The EPA continues to raise the cost of doing business in America.