Which is exactly the point. The IDW Comics Annual showed that when Sunset Shimmer was Celestia's student, she repeatedly demanded that Celestia make her a princess. To quote Douglas Adams: "It is a well known fact that those people who most want to rule people are, ipso facto, those least suited to do it."

True, but to paraphrase an anon on DP, in the past, Twilight has expressed neither the desire nor aptitude to be a princess. If Celestia didn't foist it upon her, she likely would have never even inquired about it.