I was in Japan during the mid 80s, and those JWs were really working hard. I had several calls, and even met with some ladies for a few months. (Non-JW Japanese complained about how the JWs would bring their small children for sympathy tactics.)
Japan is a country of religous fashion. I remember having coffee in a coffee shop and a long train of folks wearing white clothes, beating drums, went by on the sidewalk. Some sect of something that was worshiping a snake. I remember hearing that people usually stuck with a religion for about 2 years, then changed. I think that seemed accurate.
There was a terrible group suicide by immolation that happened in Wakayama on the beach while I was there, in 1986; it was some cross between JW and Shinto or Buddhist theologies. Yeah, strange I know, but it was all over in the paper for weeks... and people were suddenly asking questions, because most people thought it was more JW than whatever else it was, because the leader had a JW background.
I think that that played some part in slowing down the growth, because people got much more wary and alarmed about JWs... Their reaction was like that Aum Shinri Kyo cult that had that saran gas attack in Tokyo several years back--"what kind of religion would allow this?"
So the natural fad death was hastened, I think.