Stereotypes and cliches are generally based on a grain of truth. Winston Churchill said "If you are not a liberal when you're young you have no heart; if you're not a conservative when you're old you have no brain." Liberal thought tends to be emotional, while conservative thought tends to be intellectual. (The extreme in either direction is not good.) MOst "folk" music tends to be emotional rather than intellectual and therefore appears to be liberal, or left wing. Songs about broken hearts and anger at the establishment are emotional stir up an entertain an audience, whereas economics and Darwin's theory of evolution put the audience in a trance.
Crying makes better folk music than thinking.

O'Meara