It's because redheaded women are shorthand for "sexy" and "gorgeous". Whenever you need to introduce a ravishing beauty, they make her a redhead femme fatale. It's just ingrained in popular culture.
Meanwhile, black women are disgusting and few people find them attractive. However, substitute the redhead femme fatales with black women in adaptations, yet have all the characters continue to treat them like they're still dropdead gorgeous, and slowly you program audiences to think that black women are anything other than nasty.
It's just another form of social engineering. Pay it no mind.