What’s the reason for this?

At a local station: Unleaded regular: $2.15 9/10; Diesel: $2.15 9/10.

I have never seen gasoline and diesel priced the same. Before diesel became popular as an automobile and pickup fuel, the price was way below gasoline, but went way above gasoline with additional diesel-powered cars and pickups and the great increase in numbers of tractor-trailer trucks.