>>42351766
A pony must possess the moral quality associated with an element in order to wield it. This means that by the time of Nightmare Moon's banishment, Celestia was kind, generous, honest, loyal, magical, and laughterful(?) enough to accommodate all six at once. I don't see why you'd want to invent five other ponies from whole cloth just to deprive Celestia of being the moral ideal of Harmony.