Usage is low enough and destinations are various and sparse and distant enough that sufficient coverage to be useful would be prohibitively expensive, while incredibly under-utilized. You really need something more on-demand that will take you door-to-door.
Far too expensive to operate a sufficiently useful transit system for insufficient ridership to support it. The density does not allow the economics to work out.
Busses don't get cleaned often, lack modern creature comforts like leather surfaces and music, have distractingly bad noise/vibration/harshness, etc.
All of those concerns are much more easily addressed individually than by developing a vehicle that drives itself. There is no reason why a 14-passenger 8 ton Mercedes minivan could not be used as a small, relatively frequent bus as opposed to the huge, noisy MAN 40 ton chassis.