1. Why should the issue be different than for men. Since no one holds women have an obligation to spend all day learning, there’s no problem with women doing something that causes “bitul Torah”, and we have a long tradition of women working ourside the home with many gedolim having had wives who “worked” (and not just as teachers, e.g., as merchants). If college is allowed for a man to earn a parnassah, kal v’homer its allowed for a woman. Various career tracts for a woman is a separate issue.
2. Given the low status of secular women in most societies (e.g. being primarily seen as “sex objects” and the general denigration of motherhood), the outside world is far less attractive to women than for men. Looking at half-naked women is for more tempting to men, than being the one on display is for a woman. Also when a woman makes it clear she “isn’t available” (being modestly dressed and acting business-like, as opposed to social, when talking to males), that is generally respected and the person is not likely be harassed.
3. There are some all-female colleges in the United States, including at least two run by frum organizations. In addition, there is always the distance education option. And a “normal” co-ed school is less problematic for women than men.