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).