I’d take a slightly different tack as to the reasons why the Left fought in the streets during Vietnam, but not during any of the more recent conflicts. It comes down to the Draft. We had it then, we haven’t had it since. The Left simply cannot gin-up for a street confrontation with authorities because the immediate physical stakes are too high compared to potential political gain.
I would add, however, that urban riots did not result from the military draft. We don't see many of those any more either. A key factor in the disappearance of anti-military violence and urban riots is the demonstration of government force to put down both. If the government does not demonstrate an eager willingness to crush unacceptable behavior by its enemies, it forfeits its right to exist. Marching into the teeth of the national guard, throwing rocks and chunks of concrete at the guard, etc., are the sort of activity that must be crushed.
AND we should never tolerate a draft again. Nor should we tolerate the back door draft of holding reservists and national guard in combat long after their terms have expired. Our military men and women must be respected and are at least as entitled to fair treatment as anyone else.