Origins
I think it might have been more Vietnam that allowed the leftist movement to take hold.
I believe the radical Left mostly appropriated racism as an issue for political ends in the middle 60s, including groups like the Black Panthers and other such movements.
If you look at the lineup of early civil rights leaders, most of them (many of them church leaders) would be considered cultural conservatives nowadays.
During Vietnam, it wasn't just opposition to the war that we were seeing from the radical Left, but actual support for our enemies. This general trend continues on far too many fronts.