While Pokemon Go is currently available in the US, UK, and a large portion of Europe, the game has yet to launch in Japan.
CEO of Niantic (developer of Pokemon Go) John Hanke stated that the delay is due to the server capacity of Japan.
At present, the server capacity in Japan is not powerful enough. We are working hard with our partners in Japan to enable the servers to keep up with demand once the game goes online there. We expect it to be released by the end of July.
Pokemon Go players are most likely familiar with server issues, as the game’s launch in the United States, as well as recent expansions, have led to problems accessing the game. Hanke also expects the game to reach an additional 200 countries in the next couple of months.
Launching in Korea and China is a problem due to the politics of those areas, as well as limited Google map data.
Google’s map information system in Korea is limited due to security concerns over North Korea. In China, however, it is technically possible, but difficult to introduce due to the many hurdles, or should I say regulations we’d have to clear to get it to users.