Though DC was probably trying to cash in on that whole "feminism" thing with this move, it all happens because of a guy: In Wonder Woman #179, her boyfriend Steve Trevor (the male Lois Lane) gets in serious trouble and she has to help him -- meanwhile, the Amazons announce they are leaving for another dimension and that Wonder Woman must come with them if she wants to keep her powers. Knowing that Steve would die within two days if left to his own devices, WW gives up her costume and Amazonian abilities to stay with him.
Luckily, she's not helpless for very long because she immediately runs into a blind guy called I Ching who teaches her kung fu.
Wonder Woman #179 (1968)
"The secret to kung fu is -- holy shit why is my left arm Caucasian?"
A short training montage later, the new Wonder Woman is ready to go back to helping Steve ... but then Steve dies anyway, because let's face it, he was a moron. Even though her dead boyfriend was the entire reason that she didn't go with the Amazons, Wonder Woman doesn't even think to follow them into the other dimension. Instead, she opens a mod boutique, becomes a part-time private eye and starts traveling the world with her kung fu master.
Wonder Woman #189 (1970)
"I've forgotten why I decided to become a superhero!"