Cliffnotes: Japan's economy has slumped into recession in recent years (well, for a long while now, actually) and by doing so the spending power has shifted. In the "good old days" Japan didn't make content for the world, they made content for Japan, and it just so happened that what sold well in Japan sold well everywhere. The slumping economy has made it so that luxury spending by the common class has become extremely low.
This doesn't affect the Otaku, Yankii, and other marginalized minority spending habbits, however, as their spending on their hobbies has always been disproportionally high. However, with these groups still spending a lot of money, and the average person not spending very much at all, it has meant that these marginalized groups now control Japan's output. Their tastes are catered to predominantly by the entertainment industry where they spend the most money.
Hence, what Japan puts out now, only vocal minorities in Japan actually enjoy.
