I think part of the reason the Doors' "Backdoor Man" is preferred over Howlin' Wolf's version is marketting. Although I think the Doors had a certain amount of integrity as a band, they were produced, packaged, and sold to a primarily white, primarily adolescent, popular audience. When Howlin' Wolf was recording, blues was still "race" music not accessible to a popular audience. A lot of people did go back and listen to the original but I think there are still a lot of Doors fans that don't know it was recorded by Howlin' Wolf and written by Willie Dixon.