"Because we had nowhere where we could contain her power. The cell she was in was lined with the same runes that the anti-magic armour was made of, and they restrained her with metal bands covered in those runes. Even with all that, and over a millenia of starvation, I could still feel how powerful she was. Until we had somewhere to put her we had to keep her here, although I have the feeling Celestia might not have been as honest about that as I thought she was."