If we are talking about the 70's, there were several eras in that decade, including the creation of AC early on, the separation of Top 40 from Rock about the same time, the beginning of all-gold stations (as early as 1968 in DC with WMOD), the modernization of country with a wave of artists, many of whom crossed over, and the further progress of r&b after the initial Motown movement. And, of course, later in the decade, disco.It's possible that the samples I saw at the time were not representative of the country as a whole but if that isn't true, where did young women go other than Top 40? These people were raising kids. The last thing I think they would want is more noise and repetition!
You can't assume that all women in the under-40 age listened to pop music. And you have to remember that "chicken rock" AC began early in the 70s and targeted, as a core, young housewives and working women. In fact, when I was at WERC in Birmingham in 1972 there were a bunch of use who talked regularly... we had a little network with folks like Bill Tanner at WJDX in Jackson, and we also watched the more gold driven ACs like WGAR in Cleveland with Lund doing the programming.
Also remember that early in the 70's Arbitron became the standard, and we saw agencies do more and more targeted adult buying and that meant making CHR more appealing to adult women and not teens. And that is the target of CHR even today.
So it wasn't just Top 40 and Beautiful music. It was a whole bunch of formats all looking for 18-34 women.