No one knew the real reason Equestria needed a new princess, especially not Princess Twilight. No one knew, but Celestia. You see, Princess Celestia was growing very old. Her age was not outwardly noticeable, but her diminishing power was. She was not the alicorn she was a thousand years ago. Back then, she could defeat any manor of villain which threatened the land. Occasionally she needed help, as her sister and the usurping of Discord, but legend fables her conquering of countless enemies to Equestia. She still had power in her last days, but it was not enough to turn Discord back to stone, fend off the onslaught of the Changlings, or send Lord Tirek back to Tartarus.  
Though Luna was only slightly younger than her elder sister, being imprisoned within the ageless rock that is the moon and possessed by a dark force had kept her mortal soul in stasis. And Princess Cadence was hardly older than the average pony when she was appointed. But the high ruler was reaching the end of her time.  
After King Sombra was eliminated and Princess Cadence was reinstated as heir to the Crystal Empire, Celestia decided Equestia was in dire need of a fourth (soon to be third) princess. She still had told no one of her state until after Tirek was expelled; she did not wish to worry her sister, her protege, or her subjects. Only then, she entered her last weak months. And everyone began to notice when she began to let go and cancel the spell which masked her age with a youthful appearance. She had a hard time getting around, she became senile and confused, and by the end she had said her last heartfelt farewells to all her friends. With tearful eyes, Celestia had finally moved on, leaving the now three princesses, and the nation, in a state of heartbreak.  
After over a thousand years, the rightful Queen of Equestia is dead. Long live the Queen.