A beautiful woman might be single for reasons such as not being able to meet her aesthetic equal, or not being able to attract a man who is interested in her for more than her appearance. She may also be single because men are insecure themselves and her beauty.
A woman is more likely to choose a man who is roughly her aesthetic equivalent, according to research. Since most people possess average looks, it makes finding a partner easier for an average looking woman than for a very beautiful woman. This is because the number of very good looking men in a pool of single men is less than the number of average looking men. Therefore, a beautiful woman may have less of a chance of finding the right partner and may remain single.
A beautiful woman may attract a man that is not interested in who she is as a whole person, and is instead more interested in her only for the look of her body. A beautiful woman may prefer to remain single than to be desired for her looks alone.
Because a man can feel intimidated by the beauty of a woman, she may not gain his interest. Some men cannot see themselves with a woman who out rates them in looks. A man may want a woman who he feels equal with in terms of looks and for this reason may write off having a relationship with a beautiful woman.