This brief history is interesting but doesn’t explain when the state-specific cut offs were created.
Even though the PSAT was first used in 1971 the National Merit Scholar program existed before then.
http://collegeadmissions.testmasters.com/psat-nmsqt-history/
I suspect the state-specific cutoffs are a fairly recent invention as a way of increasing fairness. I doubt very much that there were state-specific cutoffs at the beginning.