Because of biology, it makes more seonse for the mother to be the primary caregiver (everything else being equal). Two reasons that I can think of:
1. The mother has to take time off (probably at least 2 weeks or so), to recover from childbirth. So there's more of an impact on the primary earner's career if that's the father.
2. If the mother breastfeeds, she spends some time doing that. If the father breastfeeds, the mother spends some time pumping the milk, then the father spends some time feeding the baby, so the time spent feeding the baby is approximately doubled.
1. The mother has to take time off (probably at least 2 weeks or so), to recover from childbirth. So there's more of an impact on the primary earner's career if that's the father.
2. If the mother breastfeeds, she spends some time doing that. If the father breastfeeds, the mother spends some time pumping the milk, then the father spends some time feeding the baby, so the time spent feeding the baby is approximately doubled.