I was aghast to read in his Depressions: their Causes and Cure, that Rothbard seemed to point to WWII as the factor that put an end to it. Doesn’t that explanation fall for the broken windows fallacy? I’m sure his explanation is more sophisticated than that elsewhere however.
I want to say that it was the repeal of several New Deal policies during the late 40’s and early 50’s, the “last hurrah” of the Old Right.