Farmers aren’t using regular diesel, they’re using “off-road” diesel, which has no taxes applied, so it’s $0.30 or more cheaper than the diesel you buy at the “regular” diesel pump. (It’s also dyed red, so if the revenuers put their stick in your diesel truck’s tank and it comes out red, you’re in deep doo-doo.)
Personally, I don’t see much future for gaseous engines in ag. We went down that road with propane from the ‘50s to ‘80s, and diesel curb-stomped all the other fuel types.