The setting allowed him to invent all different kinds of imaginary creatures, myths, and gods. Middle Earth gave him much liberty and he could create practically everything from scratch. A real place would limit him in many ways and the story would not look that magical.
The degrading show and presentation of slaves--Soloman Northrup's description of the slave auction shows his disgust for the examination of slaves as if they were horses on parade. The act takes away humanity and creates property.
Northrup describes slaves having to open their mouths to be inspected. Buyers would inspect the bodies of slaves looking for marks or physical attributes they needed in a slave. The process includes cleaning up slaves to help improve the value.
The answer I think is Captive Africans.
Belief in progress, a more secular outlook, and the importance of the individual
