One major factor: women entered the workforce. Labor supply doubled, and two incomes per household became normalized. Our current economic system fails to account for the work of raising children which was implicitely built into the "traditional family" model.
That's a double whammy for workers. The value of labor is halved. Both partners are expected to work to achieve a similar standard of living. And, without one partner doing household and child-rearing labor, those costs are borne by the workers.