In 1925, the use of five-man rotations surpassed the four-man method for the first time. It took longer than that for them to fully catch on, though, and it’d be disingenuous to suggest it’s been almost a century since the blueprint for starter usage was tinkered with—it’s a particular kind of baseball science, with change coming in waves.
Many teams reverted to using four starters by the 1950s, at which point lights had been installed at every stadium (except for Wrigley Field), allowing doubleheaders to be largely cut from the schedule and thus giving teams more time to rest their best arms.