The reduction in the length of the workweek in American manufacturing before the Great Depression was primarily due to economic growth and the increased wages it brought

See Hours of Work in U.S. History by Robert Whaples of Wake Forest University. Excerpt:

"Historically employers and employees often agreed on very long workweeks because the economy was not very productive (by today’s standards) and people had to work long hours to earn enough money to feed, clothe and house their families. The long-term decline in the length of the workweek, in this view, has primarily been due to increased economic productivity, which has yielded higher wages for workers. Workers responded to this rise in potential income by “buying” more leisure time, as well as by buying more goods and services. In a recent survey, a sizeable majority of economic historians agreed with this view. Over eighty percent accepted the proposition that “the reduction in the length of the workweek in American manufacturing before the Great Depression was primarily due to economic growth and the increased wages it brought” (Whaples, 1995). Other broad forces probably played only a secondary role. For example, roughly two-thirds of economic historians surveyed rejected the proposition that the efforts of labor unions were the primary cause of the drop in work hours before the Great Depression."