Why? Because Twilight not only became a princess at the end of the episode, but remained that way without even revealing details about her new lifestyle until the Season Four premiere, which is a long, long ways from now.
Meghan McCarthy tweeted that the S3 finale will be part one of a three-part story-arc, with the S4 two-part premiere being the conclusion of that arc, right?
I hope that in that S4 premiere, they continue the tradition of making the season premiere the "save-the-world" story, while setting it up so as to make Twilight sacrifice her princesshood, wings, and subtle growth-spurt to save her friends, just so that everything could return to the status-quo. Yes, it'll be a cop-out, but I'd rather have a cop-out than a change that could damage the entire show, especially if it remains permanent.
Plus, I liked it when the Mane Six was two of each species, including two earth ponies, two pegasi, and two unicorns. You turn one of them into an alicorn, and you would upset that balance.
And, I'd rather have the upcoming Equestria Girls spin-off and its humanized ponies, than Princess Twilight. At the very least, human Twilight gets to stay with her friends while remaining their equal, rather than be forced to leave them (and her old life) for whatever the hell her "destiny" is, like pony Twilight before her.