One theory by Linda Sax, Associate Professor at the Graduate School of Education & Information Studies at UCLA

From an article Sax wrote for the Chronicle of Higher Education:

"We have indeed reached a critical juncture in the history of women and men in higher education. Today — decades after the women's movement started what became monumental gains for female students in terms of access, equity, and opportunity — the popular notion is that gender equity has been achieved. Some higher-education statistics do paint a rosy picture for women, who now are the majority of undergraduates (about 58 percent nationally), earn better college grades than men do, and are more likely than men to complete college.

Although it is easy to view those facts as a clear indication of the progress made by women — and of the challenges now facing men — interpretation of them depends on how deeply one looks. In fact, the real story is not that men are not going to college. Rather, college enrollments are on the rise for both genders; it's just that women's enrollments are rising faster than men's.

As reports by the American Council on Education and the Education Sector have told us, the growing gender gap in college enrollments is attributable primarily to increases in college attendance among women from groups historically underrepresented in higher education — namely, African-Americans, Latinas, older students, and those of lower socioeconomic status — while enrollment gains among men in those groups have come at a much slower pace. The significant gender gaps in education among those groups show that we need a serious examination of the forces that keep men out of college. The gaps also have important implications for colleges, as female students and male students are becoming increasingly dissimilar."