Very interesting. I'd guess it's because the Japanese are more educated (at least that's the way it seems to me). I think it's indicative of what many of us have said on this site - that JW recruiting among the educated is now having almost zero success, but any growth is in developing lands where the education level is low.
JWs had success a few decades ago in Japan for the same reason it had success with some of us. Knowledge was not as widely available at that time. Also, the timing was right for JWs. During the 70's there was the supposed energy crisis. There was the cold war. The time was right for an end-times group like JWs. JW doctrines seemed more reasonable than many of us those taught by churches. It seemed to make sense.
But then... this system kept on ticking. Time ran out on another JW prediction (that the end would come before the 1914 generation died off). Material showing the real history and wrongs and flaws of JWdom became available on the internet.
The less educated, naïve, and ignorant were not as affected by this for one or more of several reasons - they didn't have the ability or resources to avail themselves of the material, they didn't have the mental capacity to comprehend it, and/or they just blindly trusted the org. But many of the Japanese are more educated and they have the resources to avail themselves of the material and they have the smarts to process it.
I would guess that there's almost zero growth in Japan from the outside as there is in the U.S. and other such places. Also, a lot of the born-ins are leaving. Maybe even some of the old-timers are waking up.
So, no growth from outside and some on the inside leaving = loss.