I realize this is a controversial question, but this post by The Diablogger has forced it to the front of my mind, and I'm not one to shrink back in fear of the politically correct crowd. There are three possibilities that I can see:
1. Beauty and intelligence are both rare. If, say, 1 in 100 girls are beautiful and 1 in 100 girls are smart, then only 1 in 10,000 girls are both smart and beautiful.
2. Beautiful girls have learned that they can get whatever they want by using their looks; even the ones with the potential to be intelligent have not developed it. They have invested their time cheerleading rather than studying, and reading YM and uh... Cosmo (?) rather than Dostoyevsky and Tolkien. This is, in some ways, a function of our culture which values the appearance of a woman more than her brains; such a trade-off will not benefit an intelligent woman in the long run, however, since by neglecting her brains she is only increasing her attractiveness to unintelligent men. Intelligent men who are attracted to her will lose interest quickly enough.
3. Maybe beautiful and intelligent women are everywhere, and they avoid me. I just threw this in for completeness -- I think we can safely rule it out.
Whatever the reason, brilliant and studly men such as myself have a hard time finding women who are worthy of us.