Beauty and attractiveness can be defined in so many ways. Ultimately, its truest measure is defined by what that person is looking for in a mate.
To answer the question though... I have to agree with hotmilf's theory 1. It has been my experience in talking with a lot of extremely beautiful people that they sometimes are lonely - many times, to be extact. People have a tendency to sell themselves short and choose not to engage in conversation with a person whom they think is extremely attractive for fear of rejection or the "out of the league" syndrome. That is too bad because possibly... just possibly... these people are looking for someone who is an average "joe/joanne" and who can relate to their feelings and their insecurities, too. Afterall, we are ALL human and we ALL deal with many of the same issues - daily.
I know lots and lots of pageant girls who've told me this. My wife is an ex-pageant girl who conveyed the same comments. Her cousin, who is presently single and gorgeous, likewise experiences the same type of lonliness. I suppose we need to be more confident and just bite the bullet, so to speak. Who knows where things could land.....