There's a few things going on that come to mind. Big trucks often have other systems that historically can be dependent on having a good consistent power draw

There's a few things going on that come to mind. Big trucks often have other systems that historically can be dependent on having a good consistent power draw source. Also, air brake systems; The brakes on a power unit are designed to 'fail safe', so rather than using hydraulic fluid to close the calipers, uses air pressure to keep the calipers open. There's a bunch of efficiency implications here but in general you either have to make a work around to that or take efficiency losses of a PTO or electric compressor. Also, not sure whether a different approach requires DOT approval or not. 'What about all those wheels and regen braking' one might ask? Then you'd be talking replacing/retrofitting trailers which has a bunch of implications for an owner/operator. Oh. Transmission losses, and the realities of a diesel; One thing that really helps the Prius (and vehicles that use similar arch) is that it (and many/most of the more efficient hybrids) use a Fixed-Ratio transmission; The engine uses various techniques to maximize efficiency at a wide band. Diesels, on the other hand, are often most efficient within a (relatively) much more narrow power band... Also in general, rig diesel engines aren't exactly designed for 'stop-start' patterns. Then, you've got the delta-V problem; The more weight you add the more energy it takes to move stuff. My SWAG (i.e. completely uninformed and friday night math) is that a power unit would have to have a battery with the capacity of a plug-in hybrid to provide meaningful value. Then, drag coefficient. A truck has a much larger profile from every angle which has a sizable impact on efficiency. Oh, and make it all with expectations for long service intervals.