- rapidly aging population (senior citizens don't buy anything, don't participate in labor force)
- women with a job have zero incentive to get married or have kids because of many cultural restrictions on married women (historically they lose their job, and husbands are home very late.) They buy their own condos now and stay single.
- consumer companies missed the Internet boom (Apple stole Sony's lunch)
- Japanese are legendary savers, not spenders. (It's considered shameful to not have $100k in the bank at all times.)
- govt is holding trillions in corporate stock and bonds to prop up their capital markets.
EDIT: this thread has some insight. https://news.ycombinator.com/item?id=26171294
Japan had the same thing happen that we're seeing now in the 1980's -- and never recovered.
Low interest doesn't help if you can just send all that money overseas.