In general, there seems to have been more interest in redheads in American popular culture in the middle of the 20th Century.
The Cult of the Blonde really got going in the 1960s, when Sweden was at its peak of fashionability in America, and tanning became trendy. The Southern California outdoor/beach/surfer/bikini lifestyle of the 1960s and 1970s was unfeasible for redheads (sun blocks weren't as effective then), while it made people with light brown hair turn blond in the sun -- i.e., their skin got darker and their hair fairer at the same time, which was a fashionable look for awhile, e.g., Robert Redford as the Sundance Kid or Sports Illustrated swimsuit models like Cheryl Tiegs and Christie Brinkley. Natural redheads got left at home, literally, by the beach lifestyle trend of the era.
After awhile, it became apparent that tanning causes wrinkles, so tanning went out of fashion (although that hard-earned knowledge seems to have been lost on the new Jersey Shore generation), but redheaded women didn't make a particular comeback into trendiness as in the mid-century.
One theory I have is that in the middle of the 20th Century, the existing hair dyes worked better for brunette-to-redhead transformations than for brunette-to-blonde. (Platinum blonde bombshell Jean Harlow's dyeing regimen in the 1930s eventually destroyed her hair and she had to wear a wig.) So, perhaps attention-seeking brunette women in the mid-20th Century were more likely to wind up redheads than blondes, which in turn generated a lot of buzz about what that special something was that redheads had that made them so fascinating. When, in truth, what it really was was that they wanted to be fascinating, so they became redhead as a way to distinguish themselves from the brunette masses.
Since then, it's become chemically easier for ambitious women to go blonde, so they do.