I have only a passing interest in this but, basically, different oil fields have different percentages of different weight hydrocarbons. The oil can be refined with catalytic cracking to make more gas / kerosene / whatever. As I understand it, we're using "lighter" oil and we're focusing on making gas, so the percentage diesel in the crack spread is lower. So, there's less diesel coming out of the refineries.

This started, I believe when fracking became a major thing (the late 2000s), because shale oils tends to have less heavy hydrocarbons then the old wells.