Blaming GM (or alternately oil companies) for the demise of the Pacific Electric Railway in LA is always popular and almost always laughable.
Southern California developed in such a pattern that it was nearly impossible for any system to completely satisfy the public. There are just too many people going too many different directions in LA. It's not like NY where most of the workforce is headed toward Manhattan.
LA's current light rail (the Blue Line) is very crowded but it only goes from Long Beach to Downtown LA. The Red Line has been a very expensive project and is a political snakepit.
While the freeways are jammed, so are buses, and LA buses are far less reliable than they are in other cities.
People in LA love their cars because, most of the time, it's the quickest way to get from Point A to Point B. It may not be the cleanest however.