"GET OUT OF MY ROOM, CELLY, I'M PLAYING OURCRAFT!" Luna screeched, slamming the door on her sister. Celestia was so disappointed in her sister that she banished herself to the sun and started an interplanetary empire.
What happened to Equestria, you ask?
Well... Luna was quite literally absorbed into Ourcraft, and lives there to this day fighting not-creepers. Sweetie Belle, Sweetie Giraffe, and their robotic counterparts in the meantime staged a coup on Canterlot. Except with both sisters gone, there wasn't really a coup. Just a lot of chickens, for some reason.
Equestria has since overthrown the Interplanetary Solar Empire because it was too short for the Giraffes' liking, and proceeded to take over the universe. Watch your back.