Many reasons, men being the macho type is a big turn-off to most women. Women nowadays go to college and have jobs, so the traditional reason for a husband is not necessary. Most women nowadays don't want to stay home, and men in their lives who are insecure keep them from having freedom. Since love is what most women are looking for in a man, being the fact that most men nowadays are inadequate when it comes to that, they look for it in someone who is loving, caring and SENSITIVE to what they are feeling.
The sexual part of it usually comes later, because generally women who turn to lesbianism do not do it for the sex. They do it for mutual love and affection. Men are by nature very controlling, and they can also be violent. Most women expect them to be the charming prince type. The fact that men are far from being charming princes in real life, makes them go for another woman.
I am not a lesbian I know some people who are. I have been married twice and have dated many men, and can tell you they are all alike, men will be men.