[timr]

The most commonly cited reason is earthquake safety: the standards for construction are constantly increasing, and old homes are perceived as unsafe.

That said, the entire housing industry in Japan has formed around the idea that homes are disposable. Unless they're very old (e.g. traditional/antique machiya and farmhouses) or apartments in large complexes, they're not built to last. Detached houses built since WWII, in particular, are more expensive to repair than to rebuild. This is probably the real reason that used homes are viewed negatively.