Celestia and Luna weren't always alicorn princesses. When the world needed them most, the alicorns came to be. But while the Royal Sisters have gone down in history as some of the greatest ponies Equestria has ever known, another was neglected. Forgotten. But what was Celestia to do? Alicorn or not, Luna was still just a child. Celestia was only one pony, and she was all Luna had. She had no choice but to focus on her sister during that turbulent time.
Even if it meant abandoning Opaline Arcana.