Why? Crawling King Snake is a blues song. Jim could sing blues, rhythm & blues, etc. Kreiger played guitar (rhythm), Manzerick the organ (blues). Duh. You telling me Morrison didn't have soul when singing? Look past skin color.
"Crawling King Snake" (alternatively "Crawlin' King Snake" or "Crawling/Crawlin' Kingsnake") is a blues song that has been recorded by numerous blues and other artists. It is believed to have originated as a Delta blues in the 1920s and be related to earlier songs, such as "Black Snake Blues" by Victoria Spivey and "Black Snake Moan" by Blind Lemon Jefferson.
https://en.wikipedia.org/wiki/Crawling_King_Snake