Why would this be? I am eager to have an answer to this question. In the meanwhile, I would like to venture two possible theories, and perhaps they are both true. The first of these is to make the observation that there is a tendency in the televisual world for output to always seek the lowest common denominator. When anything really wonderful is produced-- unless the immediate popular response to it is positively overwhelming (say like John Lennon or Michael Jackson or Marilyn Monroe or some such phoenix), one can with fair certainty expect that in a very short period of time it is going to be almost impossible to find the product in the marketplace.
The other theory is more paranoid: isn't it just about what you would expect that the true history of the Depression would be the lost history in the very country in which there has been total idiocy and amnesia about how money and markets and poverty works. Speaking of which, Blackside also produced another stunning series--which I also got from them on that occasion-- America's War on Poverty (1995). It is an unusual thing indeed when documentary footage (essentially a montage of photographs, film, music, interviews and other forms of visual and audio evidence) can actually compete with print between covers in terms of communicating the basics of what everybody who cares about the planet needs to know.
As a totally print based person who is nonetheless trying to make the necessary adjustments to the new century, almost every time I would recommend book research over video or Internet research. But in this particular case of the Blackside documentaries, speaking as a lifelong generalist, I would say it is actually safe to use this material as an introduction to what every American needs to know about "The Great Depression" and "The War on Poverty" --two really critical topics for comprehending anything at all about blues people, the African American oral tradition or African American visual culture. Let's face it. You are never going to be able to read all the books you need to read to know all you need to know. So why not cram in a little more with documentary? But the trick is to not waste your time on bad documentary. And as it turns out, that is a trick indeed because there is a lot of very very bad documentary.
For more on the films of Blackside, go to Movie Talk at http://www.michelemovietalk.blogspot.com