@Brian G. or
@Legacy can probably correct me--but I am guessing it's a matter of separate IP licensing contracts requiring them to have different acting pools for the sake of Fan Fest Nights to be able to work for the event proper.
Know there was some things that had to happen with Beetlejuice that differentiated itself a small bit from daytime operations for the actors who played the bug man, so it might make some sense especially with the scale of the experience--that things would have to be different. I also suspect and kind of wonder, if for the case of UM that there might be some design differences that separates it from being able to blend for those.