Why haven't helicopter CAT III procedures ever been developed?
Because many copter autopilots are capable of flying a Cat I ILS to 200AGL, and then descending on autopilot into a hover at a pre-determined altitude above the runway. The pilot can then descend down to the runway surface without any forward visibility. The radar altimeter is used to judge distance to the ground. Because of the unique capabilities of helicopters and their autopilots, Cat II and Cat III are superfluous.