I guess your question is not so much why they are political as why they are left leaning. As "the world's 2,153 billionaires have more wealth than the 4.6 billion people who make up 60 percent of the planet's population" (Sorce: Oxfam 2020) it follows that performers singing of oppression and uneven distribution of wealth will be popular with the vast majority of people. Folk music is by definition music for and by the people. It is pretty obvious, to me anyway, that music stemming from a majority of people that are poor and oppressed will reflect that situation. In my opinion.