History is written by the victors, so we'll never truly know. Maybe the unions were bought out?
What's more likely however is that 40h a week seemed more reasonable than the baseline at the time.
Still doesn't make it good for the humans in general though. Time and again studies show humans have no flat productivity rate, and some studies are trying to prove that 40h a week is too much and the people don't have enough productive hours left outside of that to actually tend to family, house cleaning, cooking, leisure hobbies etc.
What's more likely however is that 40h a week seemed more reasonable than the baseline at the time.
Still doesn't make it good for the humans in general though. Time and again studies show humans have no flat productivity rate, and some studies are trying to prove that 40h a week is too much and the people don't have enough productive hours left outside of that to actually tend to family, house cleaning, cooking, leisure hobbies etc.