Why does he want her and not me?

My ex boyfriend broke up with me because he claimed to be "too busy" to handle school, work and a relationship (we're both in college) It really came out of nowhere cause he was just saying how beautiful I was and hard he's fallen for me. I guess it was such a surprise to me because I thought we had a great future together and never really got closure, its been almost 6 months and I'm still not over it. I was hoping to run into him and rekindle things but then I find out that he's in a relationship with one of the biggest tramps in school and she's not even pretty! That made me really sad because I know I'm definitely better looking, smarter and have more personality than this girl so why didn't he want to date me? I've tried to get over it and date other guys but I'm still so hung up on him, that I dont have eyes for anyone else :((((