Ooh I question I might be able to answer since some of my research is dealing with this. Unfortunately I'm on my phone so this will have to be a bit brief before I can come back and edit it.

A lot of it has do with how slaves were portrayed in plantation novels(many of them were responses to Uncle Toms Cabin). Those played up the Sambo/Uncle Remus image of slaves as jovial, childish, happy go lucky, jokesters, etc