In my experience machining plastics to a high precision tolerance is a bit of an issue. Keep in mind also, that from Eaton's production perspective, the aluminum rotors are dirt cheap. Also, expansion rates on the rotors that match that of the case is very important and so I think ceramic or plastic rotors would have issues maintaining proper clearance under all operating temperatures.
Just thinking out loud.
Dave, this is what popped into my mind as well.
One issue is the expansion rates of different materials. Having rotors and the casing from similar materials helps keep the clearances more constant and predictable.
The other issue is the high tolerance required on the rotors themselves. Not all materials can provide the same consistancy with machining operations.
Its also important to keep in mind Eaton is an OEM supplier, so their focus is durability over absolute performance. If something increased performance but decreased durability....it would be a no-go.