As others have said, GM (AND the others of the Big Three) have sat on their fat asses and not kept up with what the public wants. In fact, they have actively marketed to get people to want what they make, rather than change. I offer as proof, the Mother who uses a two-ton 4x4 (in Atlanta Georgia, fer Kryssakes) to pick up Junior from school and stop at the store for some Milk and a box of tampons on the way home - WTF is THAT?
From personal experience - just try to FIND a 3/4T SINGLE-CAB, 2-wheel drive truck! I'm sure you'd have to factory-order one, if you can get it at all. Everything is a Super-cab, if not a Crew Cab, and 4x4, A/C and all kinds of other crap that only drives towing capacity down, and the price UP.
AND THAT IS NOT THE WORKERS! They build what they are told to build.
NOW, along comes the Gas Crisis, and what can they do about it? It takes 4-5 years from design, through re-tooling, to production..... look what happened in the mid-70's. What did Detroit respond with? The Pinto, the Vega and the K-Car.... eat my shorts.
Now, look at Toyota - thirty years of building (and selling!) what the Public wants, and ONLY RECENTLY do they feel comfortable enough to get into full-sized trucks and/or "luxury" cars, and even that is only a sideline for the time-being.
GM should stand for "Getta Mind".