Gosh I think there are a lot of varied and complex reasons for this. It has been extensively discussed by us over on the Feminism chat board but off the top of my head:
Socialisation
Women are socialised into traditional roles by subtle messages from being tiny. Women may have had a SAHM themselves and see it as the norm. Moreover tradition "women's work" jobs outside the home are undervalued and underpaid so it often makes genuine financial sense for it to be the woman who gives up work.
Conversely men are socialised to believe that they can be / do anything, being a go-getter, leader, driven, ambitious is definitely seen as a positive for men whereas women are seen as bossy and the term "career woman" is used as an insult once DC are on the scene.
Men are not encouraged to take on the SAHP/part time role either, ergo it falls to women by default.
Pay differential
There is still a gender pay gap although studies do suggest this kicks in later in life so potentially after kids. However there is still a trend for women to have children with older men who will be further in their careers, thus better paid. Maternity leave even if women return to work can have a detrimental impact (although shared parental leave uptake is woefully low, an issue in itself!) so mitigate that by having one ruined career (hers) and one unaffected career (his) rather than two slightly adversely affected ones.
Biological
While only 1% of children are nursed beyond age 1 (the age that the unpaid bit of maternity leave ends) it is still relevant. Also slow recoveries from birth, birth injuries requiring minor ops etc have a small effect.
Redundancy / discrimination during and after mat leave
Anecdotally this is VERY MUCH a thing and worth raging about.
Obviously, from threads on here, there are also lots of shit men out there who do not pull their weight, and a lot of women who are trapped as a SAHP due to childcare costs etc. But for most women I know, becoming a SAHM is a free choice... on the face of it at least. How free is it really considering these factors, though?