This is no small part due to the high cost of the system and its limited capabilities compared to the conventional versions and even the EcoBoost-powered +Ford Motor Company F150.

If a report from GMInsideNews is to be believed, GM has shelved the new hybrid system. If true, GM will have to come up with other solutions to meet future fuel efficiency requirements with its big trucks. GM has already acknowledged that it is working on a fifth generation version of the classic small-block V8 with direct injection and 8-speed automatic transmissions. 

The more intriguing possibility is that GM might revive its mothballed 4.5-liter diesel V8 program. Back in 2007-8, every company building full-size trucks was working on diesel engines for the light-duty versions. Following the great recession, all of these programs were cancelled in favor of lower cost alternatives like EcoBoost.

Fortunately the engines didn't entirely go away, especially the ones being developed by GM and Ford. The Ford 4.4-liter V8 found itself a home in overseas market Land Rover products.

It turns out the GM engine was within months of launch when the program was cancelled. Almost all of the tooling was ready to go but GM management decided to mothball it instead of scrapping it. Two years ago a media drive for the 2011, GM heavy-duty pickups, the chief engineer on the engine told me it could be revived in fairly short order and in testing, engineers were seeing at least 25 mpg combined with the engine. http://green.autoblog.com/2010/06/11/general-motors-4-5l-light-duty-diesel-v8-was-on-track-for-25-mpg/

That's 4-5 mpg better than the hybrid with better towing and payload capability. Combine that with the new 8-speed and the numbers could be even better. Given GM's recent renewed interest in diesel with the upcoming launch of the Cruze diesel, I wouldn't be surprised to see the new Silverado and Sierra offer this impressive diesel as an option. http://green.autoblog.com/2007/08/25/details-of-gms-new-4-5l-duramax-diesel-v-8/