Why was DW Griffiths film Birth of a Nation so successful when it was released in 1915?

It was the first 12-reel film ever made and, at three hours, also the longest up to that point.

Why is it so controversial now?​

Birth of a Nation uses explicit racism as the film provides a highly subjective history of the Civil War, Reconstruction, and the rise of the Ku Klux Klan. The film also heavily used black face on actors. Birth of a Nation gave people of colour a bad reputation to the point where white people would beat up and use racial slurs against them.