Well, there is more of an evolutionary imperative for the mother to stay and nurture her child rather than the father due to the fact that the mother has a relatively limited number of gametes so she must nurture her young, whereas a father has practically an indefinite amount. The mother would have to adapt to be the natural nurturer.

Of course, you could make the counter-argument that the way that society is structured today the father may in fact be the better nurturer.

However, from the biological facts I have read it seems to me that the mother would be better in a natural setting to raise her young. There is circumstantial evidence for this. But this is just a generalization on my part. It is a theory I subscribe to. I never stated it as fact that the mother was better. I simply said that your statement was disputable.