Remember back in the 1990s and before, diesel was always less than gas... Diesel has a long shelf life compared to gas. Why then is diesel so much more than gas? Is it because they can charge more? I still don't know why the prices flipped on which was more $$$$. A friend who is retired now worked for Exxon in research. According to him, diesel should be a lot less than gas. So what happened? Was is corporate greed that said diesel is mostly paid for by companies, so we can make up the price on it, because it will just get passed down?
I think diesel demand has increased significantly since the 1980s.
Diesel didn't surpass gas engines in big rigs until the 1970s. Diesel pickups were not really a thing until the 80s, diesel cars did start to show up in the late 70s but with the exception of Mercedes Benz most diesel cars were not very popular (and I'm pretty sure sales of diesel MBs were much smaller than gas powered).
Ford and GM introduced diesel pickups in 1983, Dodge introduced a diesel pickup in 1989. These were work trucks, soccer mom's wanted nothing to do with these slow, noisy, soot blasting beasts.
VW turbo diesel cars had a brief spurt of popularity in the 2000s before emissions regulations caught up to them.
These days you never see a non-diesel big rig and diesel pickups have become trendy, with many people buying them for status, not because they actually use them for doing truck things.
Many states also have a higher tax on diesel than on gasoline, and my understanding is that heating oil production competes with diesel fuel production which can raise the price of diesel in the winter time.
Fuel refining is vary advanced these days with refineries being very capable of breaking down less desired petroleum products to their base molecular structure and reformulating them into more valuable products. A lot of historically cheap petroleum products like naphtha and kerosene have seen price increases because what were once basically waste products are now made to meet the demand for them, any excess can be used as fuel stocks for other products.