Pokemon GO has been a wild success up to now. Since its release, it's been downloaded more than 7.5 million times in the US alone. After the initial release in Australia, USA, and New Zealand, the game has been made available in many European countries including Germany, UK, and Italy and it's expected to release in more than 200 countries in the coming months. Despite all that, Pokemon GO is still not available in its home country. According to what John Hanke, Niantic developer, told to Forbes, the software house expects to release the game to Japan by the end of July.
The reason for this delay is linked to the expected demand in comparison to the current state of the server infrastructure. Apparently, Niantic can't reasonably expect that the Japanese servers, in their current state, will be able to sustain the influx of players that the release of the game will bring. As a result, Niantic and Nintendo are currently working with their Japanese partners in order to upgrade the infrastructure to accommodate the demand.
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
According to Forbes' sources, mid-September would be a more likely release date given the challenging nature of Japanese network infrastructure.
Niantic's worries seem well-founded considering the server issues players encountered during the first days after the app went live. Many servers received an upgrade after the issues of the first week so it's reasonable for Niantic and Nintendo to be particularly cautious when releasing Pokemon GO back home and take some extra time to make sure that everything works accordingly. Especially considering the DDoS attacks that the Pokemon GO servers suffered recently that have been claimed by the hacker group OurMine.