In a diesel engine, power and fuel economy has to do with the amount of fuel injected into the combustion chamber and has nothing DIRECTLY to do with boost. In a gas engine, it has EVERYTHING to do with boost.

The added air pressured in by the turbo allows an adequate amount of air to offset the added fuel that can be injected into the combustion chambers.

Additionally the turbo is driven by exhaust which is wasted energy.

As Jim pointed out, the nature of the turbo basically shuts it down under cruise or coast conditions because there is little exhaust to drive it under these conditions.

Have a great day,