It occurs to me now that women probably didn't need to wash their hair as often in the 40's. Regular hair washing has only really occured since the 1970's. Before then, either regular soap was used a few times a month or, just after the early 20th century, shampoo was used only a few times a year. It was in the 1970s that shampoo use became prevalent (according to Wiki). Regular washing means that you wash away the natural oils (sebum) that your scalp produces causing the sebaceous to produce oil at a higher rate and so the whole thing becomes a vicious circle.
As for my own personal experience all I can say is I'm just relived I finally have that clean hair feeling!