An interesting question. I think the answer is - 9-ball happened. In the straight pool era players were mostly taking short close shots and getting down to chin level was a waste of energy. At least guys like Mosconi were mostly taking short close shots, and he and other top 14.1 players were the ones others imitated. My theory anyway.
Getting down low is better for long shots, and staying up higher is better for visualizing rebound angles, especially from close in.