"During the period from 1936 to 1950, National City Lines and Pacific City Lines—with investment from GM, Firestone Tire, Standard Oil of California, Phillips Petroleum, Mack Trucks, and the Federal Engineering Corporation—bought over 100 electric surface-traction systems in 45 cities including Baltimore, Newark, Los Angeles, New York City, Oakland and San Diego and converted them into bus operation.
"In 1946, Edwin J. Quinby, a retired naval lieutenant commander, alerted transportation officials across the country to what he called 'a careful, deliberately planned campaign to swindle you out of your most important and valuable public utilities—your Electric Railway System'. GM and other companies were subsequently convicted in 1949 of conspiring to monopolize the sale of buses and related products via a complex network of linked holding companies including National City Lines and Pacific City Lines. They were also indicted, but acquitted of conspiring to monopolize the ownership of these companies."This is one conspiracy theory that I lean toward believing. (Wikipedia Article)