For years now, Japan has been an underwhelming market for a majority of US movies when you compare it to the size of its population and its economic weight in the region.
At some point it was the most important asian market for US movies, now it's barely third or fourth after China, South Korea and even the Philippines for a lot of properties. Those markets are expanding and increasing year in, year out while japan, like Europe stabilizes or regress.
In a few years, India will probably overtake it too.