Why are diesel prices so much higher than regular gas?

Historically, diesel fuel was priced lower than regular gasoline, with the exception of exceptionally cold winters when demand increased. However, this pricing trend began to reverse around September 2004. Since that period, diesel prices have become significantly higher than those of regular gasoline. This shift marks a departure from previous market dynamics where diesel remained the more affordable option for consumers, except during specific seasonal peaks driven by extreme weather conditions and higher fuel demand.