The size "excuse" is often brought up, I don't think that's valid though, e.g. Sweden with a significantly lower density has much better public transport. Or if we talk absolute size I think even Russia has a better rail transport system than the US for example. Like usual I think it can largely be attributed politics and to the strength of the car lobby in the US (as well as a weird desire to "stick it to poor people"), which caused a complete focus on individual travel.