No 'conspiracy' just supply and demand.

Capacity to produce diesel is low, but demand is increasing. Meanwhile actual demand for gasoline is decreasing. Gas prices may go down, diesel stays the same or goes up. Because diesel used to cost less than gasoline, the perception is that diesel is unnaturally (or artificially) high.

Production capacity problems are also exacerbated by inability to distill high-sulfur crude into low-sulfur diesel. It can be done, but the refineries to do so are few and far between, so prices on low sulfur diesel go up and stay high.

This is a problem for the "drill here, drill now" crowd to reduce US fuel prices. An overall increase in domestic supply COULD reduce prices, but it probably won't have great effect since it'll be an incremental drop in the bucket on the global market and refiners will sell where they make money at the rates that give them the most profit. You won't see any "Made in USA" discounts on diesel or gasoline.

Prices aren't driven by where the crude comes from, but where the demand for the refined product is.