Housing costs are one big factor. The average middle class family with one wage earner could afford the mortgage on the stereotypical Levittown type house decades ago. Now, affording the mortgage on the same type of house typically takes two wage earners.
And this is not true everywhere in America. But is true in enough of our major cities, that both mom and dad need to work to make the mortgage.
There’s a cause & effect vicious circle in play.
In the big cities, everyone needs to work; because everyone else is. The “poverty line” today is about where the median family income was 50 years ago. However, in big cities, you’re bidding against everyone else for location — in particular, a location reasonably near to work. Two-income couples are, in effect, spending that entire second income to buy proximity to work.