"In 1974, psychologist John Voevodsky tested a small, inexpensive gadget that would eventually make U.S. highways much safer," according to a research article cited on the the American Psychological Association website. "The gadget was a third brake light, mounted in the base of rear windshields so that when drivers pressed their brakes, a triangle of light warned following drivers to slow down."
I would guess because of the low profile of many cars. If you follow one too closely and they hit the brakes, the one in the back window can be seen. Where the conventional brake lights are hidden do to profile differences.
I can recall when motor vehicles didn't have brake light or turn indicators. Some still had mechanical brakes (as distinct from hydraulics) Yep, I'm an oldie. You used to have to be sharp, really on the ball. Hand signals for turns (the driver's window was always wound down) Big trucks had wooden broom handles with plywood hands on the end of 'em. No automatic transmissions either, all manual-gear changes with no synchro on first.