Probably considered a bit small for CX; the 789 is "only" around the same size as the 330 and the 787-10 hasn't been launched yet; the 350, on the other hand, offers plenty of growth opportunities, as well as a potential 773/77W replacement down the road (350-1000).
Of course, you can be sure that Airbus sees CX as a potential 388 customer, so they will do all they can do reel them on that front as well.