> why did it get going in earnest in the mid- to late-1970s
> and not ten or twenty years earlier?
Because rates of profit and wealth concentration at the top were getting squeezed by workers, social activists and social democratic policies. So capital did the two things it always does to maintain profits and concentration: it innovated around the rules and/or outright rewrote the rules in its favor. That said innovation and pro-capital policies led to crisis is the bases for all left theories.
What's the big mystery?
Brad