As I understand it, for a variety of reasons, the value of a home tends to fall much more rapidly over time in Japan compared to the US. This probably means that if you're going to get a loan to buy a house in Japan, it's probably going to be for a lot less (relatively) and/or for a shorter period of time than you could generally get in the US, which will put significant downward pressure on home prices (again, as compared to the US).

Put another way, the more people are spending razing and rebuilding on a given supply of land, the less they'll be able to spend on the land itself.