Because, and I don't blame you for not knowing, because the game didn't really explain it, though there were hints, Nightmare in SoulCalibur V was using Raphael's body. So there's no duplication there, except for Raphael, but the story would have had Raphael get his body back and search for Amy once he regained consciousness of self, had the story been completed. So it would be a Siegfried/Nightmare from SoulCalibur I-II situation for Raphael.During the timeline of SCVI Nightmare and Siegfried did not co-exist, yet we still have them available as 2 separate playable characters. So once again, why not giving Amy and Viola the same treatment? At the end of the day I just want Viola's fighting style to make a return as it would be a shame to leave it simply wasted. For all I care, it could even be an entirely new character taking the place of Viola.
Edit: Typo fix.