It was a flat red for years because that was the only option they really had. Colors used to be extremely limited--the next step down would have been 50% magenta and 100% yellow (as opposed to 100% magenta and yellow). Colors needed to be made up of 25%, 50% or 100% cyan, magenta, and yellow. There was no way to make it more subtle.
John Romita (who defined her look) drew her hair mostly black with a small highlight, which was colored flat red. Because it was mostly black, MJ's hair looked more of an auburn and less like Ronald McDonald's hair.
But successive artists have strayed very far off model and nobody in editorial is enforcing any kind of standard appearance for much of anybody--gone are her arched eyebrows, her dimples, the cleft in her chin, her strong jawline and the heavy blacks in her hair which worked with red highlights. Once her hair became open for color--the red looked too garish.
I blame society.