"But Griffiths’ film has almost no truth in it — and it is monstrously, numbingly racist. Its black characters are drunks and rapists, played by gurning white actors in blackface, while the Ku Klux Klan are shown to be saviour knights of the new republic. Think again of that strange lack of films about American slavery, and wonder how long it will take for cinema to exorcise these demons. In that light, 12 Years a Slave isn’t simply a masterpiece, it’s a milestone. This, at last, really is history written with lightning."

http://www.telegraph.co.uk/films/2016/06/03/12-years-a-slave-review-this-at-last-really-is-history-written-w/

Griffith’s film (The Birth of a Nation) was not based on realism and was based on a very biased opinion that the KKK is our savior and that blacks are the spawn of Satan. The lack of slavery films are because of films like The Birth of a Nation causing so much political unrest and riots, causing less people to go and watch the movie making them less money. 12 Years a Slave is the start of these kinds of movies that challenge political ideas and touchy subjects.