They were afraid their servers would crash!
Wednesday’s launch of "Pokémon Go" was canceled after internal communication from McDonald's Japan, the game’s sponsor, containing launch details leaked on various internet forums, including 2ch, Imgur and Japan’s Reddit, TechCrunch reported. The morning launch was initially postponed to early afternoon after the details went viral. However, the companies later decided to cancel Wednesday’s launch due to concerns that the hype might overload the game.
While there is no official announcement about when "Pokémon Go" will finally be released in Japan, Nikkie has reported that the game will be made available in the country on Thursday.