Quoting bubbers44:-
"stall recovery in the 60's lower nose and max power
same for 70's 80's until the 90's.
Then you had to use max power and not lose more than 50 ft for some reason. Secondary stalls became a problem then."
I suspect, bubbers44, that the new (and IMO wrong) procedure was introduced because reduced vertical separation was also approved about that time? That factor might also have made flightcrews very nervous about 'sacrificing' any height in an upset?