It's the natural evolution of pretty bad, 3-chord tunes into ditties that can be easily remembered and performed, with positive feedback from people who like such things. Such songs usually tell an old tale or take a political stance. These often bristle with cloying false patriotism, self pity and attempts to tug at the simpleton heart. Pseudo-folk was exploited for big bucks in the 1950s and 60s. The whole movement is best understood by watching a movie called "A Mighty Wind."