Put another way: a barrel of oil only makes so much gas, lube oil, diesel, kerosene, HHO.
Those are separate markets, with the same base supply (crude)...but with separate/different demands...there is much higher demand for diesel because of its greater range of use...further, diesel is more for business; think trucking, farming, industrial production...those demands are not as varied as gasoline, which is principally consumer demand and consequently, more price sensitive...consumers will decrease use when gasoline gets high...businesses don't have that choice...