I think it's more than just the govt. although that's bad enough.
Diesels generally have a rather steep, short torque curve, thus the multiple gears in a big rig.
People like the way a gas engine excellerates, so diesel engines were tweaked to put out a broader torque curve. Evidently that hurts mileage.
I have a friend who for years drove an early Dodge Cummins, the 5.9 L. That truck gave over 20 mpg, he claimed 22 mpg on the road. The Ford 7.3 was a great engine, too. Then Ford went thru a series of "improved" engines, some pretty bad.
The latest diesels have all that EGR stuff, multiple sensors and electronic gadgets and they don't give near the mileage the early ones did. I think part of that was consumer driven, wanting a different torque response. Of course, the govt. made their contribution.
Anyway, they ain't what they used to be...
DF
Given a similar emissions set-up, I think my 6.7L Ford would match or even exceed that old 5.9's fuel economy while delivering significantly more HP/TQ over a wider RPM range and due to reversing the flow and greatly shortened exhaust-gas pathway, would continue to have the throttle response of a gasser.
