I think it's mainly about our modern gender roles and expectations, where both partners tend to work and earn a similar amount. With traditional gender norms, the husband was expected to be the main breadwinner, while the wife wouldn't earn much. So her subjective opportunity cost of quitting her job, staying at home and having children was quite low.

Not saying that one is better than the other, just that this seems the most plausible explanation I've heard so far.