Part of the delay might be locking them down for a long commitment. You'd need all three to sign on for three pictures, otherwise it's not going to work. Unless Storm sort of gets phased out in this film, but I think I'd be vital to get actors for Jean/Scott locked down for an additional 2-3 films at least. This could be problematic for Sophie Turner with GoT commitments, or Steinfeld who is very talented and in demand.