My guess:
1) Marinaro's A Dry Fly Code - published in 1950 - and his "discovery" of minutiae.
2) The "discovery" of Baetis and other BWO-type flies. I've read somewhere that back in the day, some states had very short seasons that didn't encompass many early hatches. It always strikes me that there really was no classic Catskill BWO pattern (Flick had a variant).