The market values Diesel more. Diesel fuel has more energy per gallon, and a diesel engine as run in the real world is more energy efficient (in a lab gasoline is slightly more energy efficient, but the difference is slight and the conditions needed to make this happen are rare in the real world). Thus those who use a lot of fuel - things like freight and earth moving normally use diesel engines. These are also applications that have less elasticity, so when price goes up they don't use less. (when gas prices go up a few people drive less - or buy more fuel efficient cars, people running diesel already are buying the best fuel efficiency they can for their needs).

While diesel is less refined, the difference isn't much. These days both are heavily refined products, and the cost to refine gasoline isn't very much per gallon.

The real answer: supply/demand is the key factor in the price you pay. Cost to create/build a product generally sets a floor on the price you pay: a business that isn't making money will stop making the product (loss leaders exist, but not a factor here).

Ultra-low-sulfur diesel became mandated in 2006 as the only type that was legal to sell for road use. That requires extra refining to remove the naturally-occuring sulfur from the fuel which is why the cost went up.

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.

Diesel increased in price because truck drivers couldn't afford not to drive their truck. The cartel as well as the distributors are extracting all the value they can at every step. That is why diesel has increased so much.

Probably doesn’t help that the ethanol that gets (increasingly) blended with gas is pretty cheap while the seed oils blended with diesel aren’t.

And the ethanol makes the fuel less valuable (energy reduction per litre) moreso than biodiesel.

If the demand for other fuels increases, the cost of diesel can be indirectly affected.

Jet fuel and diesel fuel come from similar parts of the barrel. Refineries can largely choose what kind of fuel they produce for a given unit of input. One barrel doesn't always split into the same fractions. There is configuration involved.

Supply and demand? Over here in Germany, Diesel was cheaper than (any kind of) gasoline for a long time, but that has also changed recently (Gemini says since Russia attacked Ukraine in 2022, so another thing to blame Putin for), now it's about 5% more expensive...

I thought I heard once it was because diesel might be more polluting than regular gas and thus has extra taxes on it? I could be entirely wrong though.

I was always under the impression that it was a less-refined fuel therefore cheaper.

I was always under the impression that a big part of the difference in cost is largely due to taxes.