My old truck driver contacts remember when crude was $30/bbl, and diesel was $1.20. Of course they think it's a conspiracy that it's been hovering around $30/bbl for quite a while but diesel prices haven't dropped much.
I still remember when diesel was always cheaper than gas, so people bought diesel vehicles because the fuel was cheaper. Diesel has been more expensive than gas for how long? So the economics changed somehow for some reason.