Scientific research requires a long preparation. Most women are unwilling to undertake it. Women in general are interested in marriage and a home. I should be inclined to say that all women are so interested. You can't blame the committees responsible for awarding fellowships and scholarships for tending to treat men as more permanent assets in the field of research.
Women hesitate to borrow money for advanced study.
They are afraid they won't be able to pay it back before they marry and don't want to saddle their husbands with a debt. If women keep in doing the work they start in, there is no trouble about paying such debts. It seems to me that there are barriers to continuing a research job after marriage. The general rule is not to do so. It is hard to get a double position for husband and wife.