Why don't we fly helicopters with yokes?
Well first off, a helicopter doesn't have either a yoke or a stick; it has a cyclic.  

Second, the original use for a yoke or a stick was to provide a pilot with a mechanical advantage in actuating the airplane's control surfaces against the force of the relative wind.  Helicopters never needed this as the cyclic actuated the swash plate on the rotor mast, changing the pitch of the rotor blades as they passed over.

Third, helicopters demand a little more 'hands on' attention when flying them as opposed to airplanes.  The helicopter pilot is required, particularly in hovering, to make constant adjustments to the cyclic, collective, throttle (if not equipped with a governor) and anti-torque pedals.  Therefore a yoke style cyclic is not useful to a helicopter pilot.

A cyclic does not necessarily need to take the form of a traditional joystick as in the example from an MD900 (Top) and an R22 (bottom).