Availability is definitely a big one in big cities. I recall seeing coverage maps where virtually all of the suburbs were severely underserved by existing taxi

Availability is definitely a big one in big cities. I recall seeing coverage maps where virtually all of the suburbs were severely underserved by existing taxi unions, since downtown was where volume was perceived to be. The whole Prop22 thing in California is another facet to the equation: ride sharing companies argue that freelance status means they have a far bigger, broader and elastic fleet, whereas a model with fixed costs would make availability look more like taxis before rideshare came along.