@Statsattack - there's a pretty useful concept in mental health: you can only control and affect your own response, not other people's.
You can wonder 'why' til you're blue in the face. She's never going to have done anything other than what she did. There's never going to be an explanation that actually helps you.
What you can focus on is changing your own reaction/response to her behavior.
Rationalizing her actions, or seeking to understand them, or trying to crawl inside her head - all you keep coming up against is a brick wall. You don't understand, it's not the way you think you would have reacted, and it's wrong of her to have done what she did.
So why are you still trying? You've got your answer. She ran. She left. She's dating someone.
The why of it doesn't matter. Maybe you will understand someday, when someone suddenly appears to care more about you than you do about them.
But for right now, you would be much better served by taking steps to understand how
you can work on your reactions - not still struggling to understand why or how she made the choices she did.