This assumes that the non-breadwinner did not work. In the vast majority, the second party, almost always a wife, was busy homemaking -- toiling from sunup to sundown tending children, making labor-intensive meals, cleaning, doing laundry, making clothes, growing and preserving food, etc. Did they get paid for their 12-hour workday? No. They significantly enabled the breadwinner to specialize in his field, but only he is seen as contributing to the economy.
Today, we've got machines to do the dishes and the laundry, refrigeration, and foreign slaves / wage-slaves to make our clothes and crops. Homemaking is less labor intensive today, and what could be more leisure time has been replaced with two-income houses.
And, for example, my great grandma was both a housewife and she had a job at a sweatshop. The single-income household wasn't really an option for the poor in the 40s, and I suspect that it's only been broadly available to the upper class.