I don 't think her ability to handle the magic itself was the issue. It's that whole adage "with great power comes great responsibility". She may not have been responsible enough. And the majority of the times we don't see the problems ouselves. Celestia may have seen too many signs that she could possibly be corrupted by the power, and so wanted to hold off a little longer until she was ready. When a master says one is not ready, their words are to be heeded. I see this as the same scenario as Ti-Lung and Master Shifu from Kung Fu Panda.