I have a theory. Cadence is Celestia's neice, as well as Luna's. Once Cadence married Shining Armour, Twilight Sparkle became Cadence's sister-in-law. That would make Twilight Celestia's neice-in-law, wouldn't it? Celestia seems to have favorisim over her neices, dosen't it? My theory is that once one of Celestia's neices dose something exceptional that she can't ingnore, she turns them into a princess for a sign of showing her graditute. That is like if your neice painted an amazing picture, and you gave them a castle and guards and everything.