Re: The Joker sized elephant in the room
There's the risk that any appearance of The Joker, even fleeting and hidden in shadow, would detract from Bane as the main villain of the piece in the eyes of the general audience.
I think of The Joker as Chekhov's gun. Don't put a loaded rifle on stage if you're not planning on firing it. The Joker is most definitely a loaded rifle.