So... What did happen in the 70's...?

[deutsey]

19. I think one thing was the large right-wing/corporate reaction

against the New Deal/Great Society/Counterculture.

That's what the Powell Memo helped to ignite: http://reclaimdemocracy.org/powell_memo_lewis/

Walter Karp in Liberty Under Siege gives an overview of how the DC political establishment (Democrat and Republican) circled the wagons against post-Watergate egalitarian reforms as part of this reaction: http://www.amazon.com/Liberty-Under-Siege-American-1976-1988/dp/0805008594/ref=sr_1_1?ie=UTF8&qid=1382636657&sr=8-1&keywords=liberty+under+siege

In Part I Jimmy Carter is described as the candidate of the democratic awakening, marked for systematic destruction by Reaction and losing one battle after another as the press grows "more stupidly cruel as Oligarchy grows more brazenly vile and Carter more stupidly weak." In Park II Karp turns his baleful eye on Ronald Reagan, whom he characterizes as "an ignorant, truthless demagogue." The story of the 1980s, in Karp's view, has been "the exaltation of a tyrant and the degradation of a republic."

Simultaneously, I think the assassinations and violent state crackdowns of the '60s and early '70s along with the growing fragmentation and self-indulgence of the counterculture ended up weakening left activism.

There are other factors (like the energy crisis), but these are my main reasons.