Nanook and Me by Louis Menand
People who make documentaries don’t make them because they believe that “reasonable people can disagree,” or that there are two sides to every question. They believe that there are, at most, one and a half sides—a right side and a side that, despite possibly having some redeeming aspects, is, on balance, wrong. They make movies because they are passionate about their subjects and they want to arouse passion in others, many others.
These passions may tend to be progressive rather than conservative because progressives are more likely to be the sort of people who feel good about expressing their activism in an artistic medium that requires hardship and teamwork, and that results in a product that has little chance of making anyone rich.
An astute point, "expressing activism that requires... team work and... has little chance of making anyone rich". Hmmm sounds like the inverse formula to punditry and talk radio.
Different forms of expression to achieve the same ends, 'Documentary' the natural outlet for progressives and Talk Radio the preferred choice for conservatives.