Supply-and-demand. As women entered the workforce, this created more supply, which drove wages down, which forced more women to enter the workforce. Similarly

Supply-and-demand. As women entered the workforce, this created more supply, which drove wages down, which forced more women to enter the workforce. Similarly, two-earner households could outbid single-earner households when purchasing real estate, which also drove the need for a two-earner household. This transition is not reversible. However, it only happens once (excepting polygamy).