ELI5:What has changed in the last 40 years so that it now takes two incomes to maintain a household?

There is a book about this, called The Two-Income Trap.

It has a lot of interesting information about the phenomenon, but the basic gist is “women got jobs, so now the market has adjusted to match those two incomes, PLUS there is real economic value in having a homemaker (cheap cooked meals, babysitting, etc).”

Explain Like I`m Five: good questions, best answers.