Dear Mama Mary -
Why do the guys that break up with me end up with these girls that, well, I'm better looking than them! And they're like a box of rocks, too. Why?
A Still Unattached Daughter

Dear Daughter -
Simple - you're intimidating them!  You're beautiful. And you're smart. Either one of those would intimidate a guy, but together? It takes a really special person to handle both. And knowing, you, Daughter, you're willing to correct ignorance, which deflates a male ego, especially an immature one. And you dress to impress when you go out, which attracts circles of admirers, which a guy views as instant competition.
(The husband said something to me on this subject recently. He says a beautiful woman really does make a man think "I'm not worthy".   [I hear it in Wayne & Garth's voices.] He says it takes a lot of ego to think you could get, let alone keep, a beautiful woman. So that's the guy's side.)
These guys realize that not only are you smarter (and more ambitious, courageous, popular, et c.) then him, you're likely to leave him because your beauty keeps an ever-refreshing pool of new guys around. The guy bails, and settles for a girl or woman that's "below" him, so he can get his ego propped back up. This poor female will probably adore him, but whatever. Not our problem.
Your mission is to find a guy that's your equal, or who accepts that you're smarter and/or more attractive than he is, who accepts your intelligence and treasures your beauty, both inside and out. They are out there, but remember my 90% rule, which means they're outnumbered by assholes. Know this, and have fun looking!