So I've been thinking about this for a while, and I think I might have come up with an answer. Celestia was buying herself time, It is very likely that she didn't have full control/power of all the elements, therefore, she could not reform her with them. It was Celestia, not the elements of harmony that sent Nightmare Moon/Luna away. So she sent her away for a thousand years, so she could, come up with a plane, find the new bearers of the elements, find a pony capable of holding the element of magic, and ultimately bring Luna back.
Keep in mind this is just what I think happened, it is not in the show. And please leave your responses, and if you find any holes in this theory, tell me so I can either patch them up or, dismiss the theory entirely.