"Because Celestia didn't make Twilight and alicorn. Twilight more or less "attained enlightenment" about the nature of friendship and ascended. Celestia only guided her to that point. She's been grooming Twilight to be her replacement. Even Lauren Faust said that she always intended for Twilight to replace Celestia. The current writers thought her ascending was the best way for her to do that. Twilight earned her wings, so to speak. Also, you may be taking this whole thing way to seriously."
And this got me thinking about the spoilers we saw at comic con depicting the return of "evil Luna" as I'm going to call her (In my head cannon the comics showed nightmare moon being being "freed" and even defeating the nightmare forces). Twilight is seen saying "where have you can Celestia dissapeared to".
Just as sylentmana said," Even Lauren Faust said that she always intended for Twilight to replace Celestia". Maybe this is the time where they decide to kill Celestia. This would definately cause for an interesting turn of events within the world of equestria, and It would defiantly answer all of my questions as to what Twilight would now have to do with her new title.
Thought this is probably to dark to be in a children's TV show, I have no doubt that Celestia will have been captured in S4E1 and Twilight will have to get her back some way. I predict this will happen weather or not the whole return of evil Luna" is a thing or a mere dream of flashback (as some have suggested), because its a great chance for Twilight to show off that, if needed she can rule equestria and that she proves Celestia right in giving her this new responsibility.
So ending on a more sensible note, I can't wait to find out if my prediction holds true . We only have a few months left now till the opening episodes, and personally I know it's going to be a long hard wait with many more wacky prediction between now and then. I hope I don't lose the plot before the next season ....