The Army has historically used SNCO pilots widely because it has needed a large number of highly competent Drivers Airframe. These drivers did not need to have a wide or deep knowledge of unit level tactics and procedures.
Though I have been out of the game for a long time, my understanding is that the AH opens another dimension and, when hunting in packs, often needs commanders with unit command training and experience. Hence the increase in the proportion of officers required in AH units. The AH can still be driven by a SNCO, in fact they will usually have more practical skill and experience of flying the aircraft than an officer who has other strings to his bow.