The question for 2008, and here is my (easy) answer:
1. Japan has the highest pro-capita rate of architects, of the 1.2million registered architects world-wide Japan has over 300,000,
2. There are very few aesthetic regulations: anything goes,
3. In the chaos architects work hard to make their buildings stand-out,
4. Tokyo has some of the world’s most daring and cash-rich clients,
5. Japanese construction companies (not only the major ones) are the world’s most advanced,
6. Buildings have an average life-span of around 30 years,
7. The city has been destroyed many times over (and most likely will be again soon).