Revenue management happened a little while later - In the early '80s, when AA introduced their super saver fares.
At the time they were faced with increasing competition from the likes of PeoplExpress whose operating costs were half of AA's. However since AA was averaging only 60% load factors some bright cookie realised that they had almost no operating costs for 40% of their seats.
As long as they could find some way of preventing the already travelling passengers to buy down they could undercut their low cost competitors and make money. And they did, for a while.