Safety.
As above, two low vis approaches must be carried out in the simulator every 6 months, leaving one to be done on the line during the interim. This can either be an automatic approach and landing done in anger as it were. Or it can be done in good weather. As each pilot must self certificate this approach, if a real Cat III approach has not been carried out, and it's getting close to sim time, someone is going to have to give away a landing to the aircraft.
And we don't like doing that do we!
