One of the rather frequent responses I get when posting the stories of people like Harriet Tubman, Frederick Douglass, or Robert Smalls is that their story deserves to be a movie. A biopic is seen by a lot of us as the ultimate testimonial to a person's life. Moreover, movies have the unique power to reach and influence millions of people. Finally, movies offer the possibility of all the imagery and input we hold when thinking of, say, Harriet Tubman to be made manifest before the world. I think this impulse is basically correct. It is especially correct given that Hollywood doesn't just ignore slavery and the Civil War but turns out revisionist dreck like Gods and Generals.
At the same time I think it's important to not talk as though it were an entity separate from the politics, economics, and history of America. The person who would bankroll a Harriet Tubman biopic would likely be someone who was particularly touched by her story. Such a person would not have to be black, but I don't know how you separate the paucity of black people with the power to green-light from the paucity of good films concerning black people in American history.