Geography 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...