Masahiro Morioka argues that Japanese herbivore men are a result of Japan's post-war peace. Since the end of World War II, Japan has not directly participated in any war or conflict, either within its own borders or outside of them. Prior to this time of peace, many Japanese felt that becoming a soldier was the only approach to becoming manly. This social norm has slowly disappeared during the following period of post-war peace. Due to this, Japanese men are less aggressive and this could bleed over into their romantic lives.
Even as late as the '80s, Japanese business culture was notoriously macho and aggressive. When I first read about herbivore men, it was framed largely as a reaction against '80s Japanese business culture more than anything else. I'd be more inclined to argue that herbivore men came out of the recession of the '90s; they saw how the bubble economy collapsed and said "screw this, I don't want any part of it".
I know I have a tendency to try and view everything through the lens of WWII, but this really sounds like after the war, the US somehow neutered the entire Japanese culture a little too thoroughly.