Probably because it would have taken an act of Congress to do so. Look at the PRNDL standardization on automatics--before that, it was any mixture of positions, with or without park, and maybe even using buttons. Took a moment to find but apparently it was "fixed" in 1961.
Link. Automakers were historically very against doing anything that impacted their bottom line--or change, really, not unless if they themselves instituted it. Over the last 50 years or whatever, just what would have been the impetuous for these automakers to get into a room and say "hey let's all have the same shift pattern?" what would it have profited them to work together on this?
Heh, I'm reminded of a story that my grandfather told me. At some point in time, he was asked to move someone's car. So he put it into reverse and... darn thing moved forward! Took him a while to figure out. VW Beatle, I'm guessing this had to be in the 1960's when this happened, and he was not the sort to have bought anything other than domestics with good ole 3 speeds at that time.