It's certainly possible that there are women who are (or were) interested in CS but decided not to pursue it because they have (had) more lucrative opportunities. There are lots of career options open to women such as medicine, law, and business, which (arguably) have more job security than CS does these days. Furthermore, perhaps these women do not find CS as interesting as men. There are certainly intellectual challenges in other fields.
Another source of thoughts on women in CS (by a woman) is Lynne Jolitz' site.