Maybe it's just that folk music has followed a long tradition (hundreds of years of borrowed tunes, instruments and, by assimilation, ideas) -- a tradition that can only grasp a relationship between the power/government in a parental/royal role of caring for the populace.

About two hundred years ago a divisive idea started to really take seed -- that the populace could somewhat self-determine. The idea that an individual's well-being, health and happiness was not the responsibility of a government of kings, but rather, a matter in which he could have voice and self-determination.....

......but you can't protest against yourself for goodness sake. The liberal mindset is still more closely tied to that of dependance on government for health, wealth and happiness. This is the perfect perspective from which to launch protest. Every problem is of someone else's making. (One reason why I like the blues -- it deals with personal problems and makes no other claim than (often humorously) self-attribution for those travails).

This is one reason that what there is of folk music now (currently written) is of a personal vein -- singer/sonwriter sap. The overwhelming direction of the government is a movement toward caring for every single need of the populace. There may be some things left on the liberal's wish list, but overall the progress of government, and the diminishing need of the overwhelmingly wealthy populace (from an historical perspective) has removed the wall against which a protest movement must reasonabely lean.

The left likes naughtiness -- but only their own naughtiness. There may be any number of personal vices for which the left would like greater liberation -- but these desires don't make for great musical themes. They come across as selfishness and are harder to be taken seriously.