My theory is that Celestia was powerful enough to take control of all the elements, but was never able to use them to their full power, hence why she could not redeem her sister, and she was therefore forced to banish her to the moon.

And you thought Celestia was just being an asshole god. Shows what you know, you judgmental jerk.

“The stars will aid in her escape.”
Four stars + Luna + Celestia = Six bearers.
Four previous bearers banished to the stars seeking revenge on Celestia by releasing Nightmare Moon?