One way to think about it is that buying a long-dated crude future is the same as buying some crude at approximately the spot price _plus_ a supply crunch option. The supply crunch option pays off if there is a refinery fire, Mideast terrorism, Nigeria war, whatever.
It makes sense that the supply crunch option premium will increase as the time to maturity increases, thus you have increasingly long dated crude futures at higher and higher prices.
If you short crude futures you are essentially writing the supply crunch option.
Aaron Schindler
Schindler Trading