In our case, I would say it's because it made more financial sense and I'm a control freak and I want to. Though to be fair, when my dd was little, it was because my husband's income was based on how much he worked (he runs his own business, fewer hours means fewer clients which is less money), whereas mine was based on a specific project. I got paid the same whether I worked 40 hours a week or 20 hours. The only difference was that the more I worked more hours per week the project would have finished sooner and I could have moved on to the next one, but I wouldn't have made more money. In terms of childcare costs, it didn't make sense for me to work more as it was no financial benefit to us. But since our daughter was about 3 and I went back to work full-tme (different job, pay related to hours worked), my husband actually has done more than me. He did the school run 3 days a week, plus all the washing, cooking, cleaning on those days while I worked. I did the same on the other days (he worked longer). I do more of the mental labour around the house (remembering things, keeping track of the diary, meal planning), but that's only because he's hopeless at that. He does the washing up every morning and evening, puts clothes away, takes the bins out, does the hoovering, does the school run 3 mornings a week (though I do the others and the pick ups most days), cooks dinner at least one night a week, mows the lawn, household repairs, etc. We both do the food shopping. So housework is shared fairly evenly in terms of work, but I tend to be the control freak so I manage it all.
Our 2nd is due in a few months. I will take off, but it's because I get maternity allowance. I don't think he can get the same. He doesn't get statutory paternity pay as he's self employed, so taking time off with baby is just that, unpaid leave. It's fine for a few weeks, which is what he takes off, but it seems silly for him to take off and stay at home when he wouldn't be getting any maternity pay. Whereas I at least qualify for maternity allowance for 9 months. After that, I'll go back to work part-time, but mostly because I want to. My work involves a long commute (2-3 hours) and lots of travel, which I don't want to do with a baby. So it's sensible (and preserves my sanity) for me to work part-time for a bit and cut my travel, than for him to turn away business. But we have a pretty egalitarian and progressive household.
I suspect in most cases it is just whats socially acceptable and also because traditionally women earn less, so financially it's better for them to cut their hours. I don't think it has to be a big feminist issue though (social stereotypes and unfair labour practices aside). It's not like working yourself to death in long hours and missing out on family life isn't liberating. I'd much rather have a good work life balance and do a bit of both and I think it's personally beneficial for me that it's more socially acceptable for women to have that. I actually feel a bit sorry for my husband that he doesn't have the opportunity, given our finances, to take more time off for family.