Fester, I was NOT around back then myself - LOL!
But I do recall my Grandpa and Dad both telling me when I was a wee lad that folks got in on the passenger side as trying to get in on the drivers side when parked on the street was down right insanely dangerous. Folks driving didn't watch out for doors opening or people trying to enter/exit their vehicles leading to a whole bunch of tragedies.
That reasoning stuck around for a long time as we see with AD trucks still mostly outfitted with keyed locks on only the passenger side doors. It was a way to try and encourage folks to unlock and get in on the passenger side to help continue preventing such accidents.
