if it is running near capacity, its crowded, and therefore unpleasant, which causes people to attempt to adjust their schedule to use it at other times, shifting the load

if its not running near capacity, its wasting a lot of space, and therefore more wasteful than private automobiles

in both cases the scale of the system and the centrally planned nature of the system make its response time lag behind the people using it, causing them to seek alternatives if they are able