There are many reasons. One of them is that they don't have adequate child care. Many women, because they lack alternative supports, especially in academia, get trapped in that contract work. That's one part.
The other part is that we simply don't have enough jobs in the areas where women have education, but also there are issues about hiring practices in terms of whether we recognize equity principles in hiring. It's not one factor, but many.
However, child care is important.