With the caveat that this facile explanation is almost certainly insufficient to explain large scale changes in the largest national economy: I believe during

With the caveat that this facile explanation is almost certainly insufficient to explain large scale changes in the largest national economy: I believe during the boom years of industrializaton, THE scarce resource was manpower. Factories were full of machines, and machines were operated by humans. In 1943 U.S. Steel employed 340,000 workers. The high demand for manpower allowed workers to capture a greater proportion of the economic surplus their activities generated through negotiation. At some point other ingredients of business became crucial and thus the balance of power shifted.