One of the primary reasons houses depreciate in value so rapidly in Japan is simply because that is the accounting rules. If you look at http://www1.m-net.ne.jp/k-web/genkasyokyaku/genka-tatemono.h... (and I hope Google Translate translates it well), you see that the typical wooden houses (the kinds you see in California, where I live) depreciate competely in just 20 years in the eyes of tax agency. This has real effect on mortgage.
The rules around the market are different, too. For example, in California most houses are sold and bought as-is. In Japan, the seller is on the hook for up to an year for problems that weren't discovered at the point of sale.
These differences depress the existing house market, and that is made up by the new house market, which in turn translates into a lot more new houses.
I find this report from Ministry of Land, Infrastructure, Transport and Tourism highly educational. It comes with lots of numbers: http://www.mlit.go.jp/common/001002572.pdf more