The change from the old days was most likely brought about by the takeover of business by MBAs, finance types, lawyers and so on that happened around the 1960s. Before that long-established companies were almost always run by people who knew the exact business the company was in. That is, they were geeks. Think Ford, Disney, HP, Hughes Aircraft, Lockheed, etc. 

Then a new theory of management appeared out of nowhere: a good manager can manage anything. In short order, companies were taken over by generic business majors, people who didn’t have a clue about the business, they just knew business in general. It became kind of quaint to think that a company should be run by somebody who knew the exact business, and computer companies thought nothing of hiring a CEO whose last job was selling sugar water. 

Was there ever a time in the US where working 9-5 was the norm?