alt Hacker NewsGeography I guess[1].
Kanto is flat, it's the only region in Japan that could sustain feeding such a massive population and could allow building the first mega city on the planet.
Combine that with the massive engineering and rail experience Japanese have, and it's no surprise imho that combined with favorable geography they could build it quickly.
[1] https://upload.wikimedia.org/wikipedia/commons/2/28/Topograp...
This is interesting analysis. Many good points. Regarding this comment: "first mega city on the planet": As I understand, in the modern era, Beijing was the first city in the world to have one million people.