Simple supply and demand.
Refineries are turning out 50+ formulations of gasoline each year as well as other petroleum products like diesel. It's not physically possible to make all of these products all of the time and new refineries are built only rarely due to onerous regulations (although CA has recently relaxed it's stance on building new refineries).
If enough new refineries are built prices for some products, including diesel, will come down a little, but not a lot.
The demand for gasoline argues for favoring the production of gasoline over diesel, leaving just enough diesel available to be sold at what the market will bear (i.e., an historically high price).
For those who allow high gas prices to lead them to the conclusion that oil companies / gasoline retailers / OPEC / etc. are somehow getting away with murder I'd suggest remembering that oil is a commodity and behaves in the marketplace like every other commodity (political interference aside).
I mean, copper is ridiculously high right now (I know because I have to buy a lot of it) and it's due to good old supply and demand. The world is presently using copper (and oil) at very high rates and producers are able to price accordingly while at the same time working overtime looking for more copper (and oil) to sell at today's high prices.
And, if demand remains constant and the search for more copper and oil is successful, then prices will come down. Likewise, if demand cools because of pricing or any other reason, then prices will come down.