It's probably a combination of several factors but I would imagine the biggest factor is that women have more non-work demands (social pressures and gender role

It's probably a combination of several factors but I would imagine the biggest factor is that women have more non-work demands (social pressures and gender roles making motherhood frequently more demanding than fatherhood, women being expected to carry a larger portion of housework, etc.) than men, and so an equivalent amount of time off is less satisfactory since less of the "free time" is actually free time. I suspect that another part of it too is that in many jobs women may feel judged more harshly and so less able to actually take advantage of the flexibility that's there since there's already a struggle to be noticed and acknowledged.