Just because Hayden Christiansen was in Vaders armor at the end of Revenge of the Sith doesnt mean he has to be the only guy to ever wear it again. If the original trilogy taught us anything, its that many people make up Darth Vader. At just 1.8 meters, Christiansen is nowhere near as imposing as Vaders original actor, David Prowse, who stands 1.98 meters. The Revenge of the Sith Darth Vader suit had to be designed to build on Christiansens smaller frame, and it would seem a little ridiculous to do so again. Christiansen has had his time to play Vader; maybe the future movies could show him as a Force ghost, but he's simply not needed here. The only requirement for being the actor for Darth Vader is to be tall, handle stage combat, and use the same mannerisms as Prowses Vader.