Diesels give better torque at lower RPM because of the benefit of the much higher compression ratio and higher volumetric efficiency. Put a turbo or supercharger on a diesel and you can have as much torque as the engine will stand, at propellor friendly speeds and not lose the economy. It isn't necessary to use a prop reduction gear assembly, unlike with some bigger piston engines, which somewhat offsets the higher weight of the unit.
Not forgetting that one day they will be filling the fuel tanks with renewable bio/vegetable oil.