my impression of why twilight is a alicorn

[Clampettie 2]

Twilight was destined to be a alicorn from the start. Her cutie mark was a star and what makes the night stars and a moon and the moon cutie mark is princess lunas  and luna is princess celestias younger sister and celestia was Twilights tutor. Thank you for taking you time to read this

 

[Twi Rubix 726]

Realistically, it was to promote the toy line, but I really like that theory. But as we saw in Magical Mystery Cure, according to Celestia, she gained lots of learning experience, and her magic really advanced, so she felt Twilight was ready.