New York City is a city that is well planned for using mass transit. It has a centralized business district allowing commuters to arrive in one primary location from a variety of arteries. Cities such as Los Angeles are decentralized and therefore are not well designed for mass transit.
Initial costs are astronomical.
1 mile of light rail in the U.S. today costs between 40-50 million dollars. Americans love their automobiles, resulting in low ridership on many mass-transit systems. This is perpetuated by the relatively low cost of gasoline in America. In Europe and Asia mass-transit is much more successful because prohibitive gasoline taxes make regular operation of a car a real financial burdon. Because of low ridership in the U.S., mass transit systems also do not pay for themselves.