I'm in my 50s, have been into this in one way or another all of my life, and definitely remember the pre-Internet days!
I've always thought that--IN GENERAL--men are into it because it's a sexual turn-on. Either it's a fetish, or we like the humiliation / domination aspects of it, or we're aroused by looking at others wearing and using diapers, etc. Women seem to be into it for the comfort aspects--being cared for by a loving "daddy" or because it makes them feel content and secure.
So it then follows that men would be more adventuresome and enjoy going out in public in their diapers for the thrill of it (at least I do!) where women would be more likely to put on their diapers, curl up in bed, and relax. That being said, it always made sense to me that in the early days, men would be more likely to write to various ABDL publications or join DPF during the pre-internet days.
But now with the internet, I think we're seeing lots more ladies opening up about their own ABDL desires. It's safe and easy for them to do so, and as mentioned earlier, I do think that society is becoming more accepting of our (and everyone else's) kinks.
But I also think that there were always lots of ladies into this--we just never heard from them during the early days.