Jesse James is done repeatedly, last time in American Outlaws, played by Colin Farrell. He has a black sidekick!
Turning Jesse James into a Western Robin Hood covers up his role as a Confederate guerrilla, who took part in massacres. Having a Southern rebel motivated by something other than racism is the same kind of falsification promoted by the NeoConfederate sympathizers who babble about economic oppression by Northern industrialists or dedication to states' rights.
You're expecting Hollywood to stick to the facts? They're not making documentaries, they're making entertainments.
In the classic Western there there are no significant numbers of African Americans or Chinese; the Indians are simply bloodthirsty marauders or noble savages doomed by the modern world or simply not part of the equation (none of which were true!); Manly Men heroically struggle with the lawless landscape, far removed from both the blessings and blights of civilization; noble settlers start anew in unpopulated territory. In various ways, this is a dream world where issues of race, from the annihilation of the American Indians to the nature of the Civil War, simply don't exist. After integration made race an issue, the mythology no longer bore conviction. Even on television, younger audiences would rather watch cop shows.
Well the classic Westerns were made in a time when Blacks or Chinese were rarely seen in any type film produced by White Hollywood. Comic relief and on ocassional villian were about the only roles offerred. Black filmmakers made their own films, including Westerns starring black actors.