It's probably a function of both population and area: higher population density makes mass transit more efficient. Europe is much more densely populated than th

It's probably a function of both population and area: higher population density makes mass transit more efficient. Europe is much more densely populated than the United States: https://en.wikipedia.org/wiki/List_of_countries_and_territories_by_population_density https://en.wikipedia.org/wiki/List_of_countries_and_territor...