I think the frequency of washing, which changed as showers became more common played a large part in it.

Prior to that, IME it was more common for people who washed their own hair or their children's hair to wash it in the sink than the bathtub. Though many of the same things apply, the water would have been cleaner. It was also independent of the rest of their bathing schedule.

Women frequently only had their hair washed in salons, when they went for their weekly or bi-weekly wash and set. Teenagers, and grown women who didn't have regular salon appointments were more likely to set their hair up in a full head of rollers (possibly with a pin curl in front of each ear, and/or for their bangs, just like in the salon, but less likely to use "setting lotion." Once the hair was combed out, liberal applications of hairspray followed. Between sets, they'd sleep with their hair covered with nets, and their bangs taped down – très enchanté.

When blow dryers arrived, it became possible to wrap your hair around a (round) brush, section by section, and really get some traction on it, pulling curl out of all but the ends, if you so chose, but it had to be done pretty much daily because of the weird things which happen to curly hair when it's slept on, and I suspect that contributed to the popularity of washing hair daily in the shower.
Peace,
Morgan

Baby Fine 3B, low porosity, normal density and elasticity
CGing since July 2008