The Port Authority which runs the PATH subway system while they accept MetroCard they have their own tap card system called SmartLink you can buy multiple trips at a discount (no discounts for Metrocard and unlimited MetroCards do not work in the PATH system).
Perhaps they might expand SmartLink to the AirTrain system, haven't seen anything about it but to hazzard an off-the-cuff guess this could be why the PA is not adopting OMNY.