content: This is pure speculation, but my best guess would be that you can get more torque to turn the wheels using the levers, because they could be exceptionally longer than a handle bar. Also with a handlebar or steering wheel you need a steering column to support & transmit the turning power to the wheels, which would make it more difficult to get in & out of the vehicle (you'd be banging your knees against the column). One more disadvantage of a steering wheel would be trying to use hand-brakes while simultaneously trying to turn the vehicle, which would be extremely difficult to execute.