Why did Japan get Pokémon Green but the West got Blue?

Pokémon began in Japan on February 27, 1996, as Pocket Monsters Red and Pocket Monsters Green for the Game Boy. There was no Blue in the first pair. Blue came later that year, on October 15, 1996, and Japanese players could not walk into a shop and buy it: it was a special edition sold by mail order through CoroCoro Comic magazine, with revised graphics, a rewritten script and remixed encounter tables. It did not reach Japanese retail shelves until October 10, 1999, almost three years after the mail-order release.

When the games crossed the Pacific, Nintendo of America chose the newer code as the foundation. Pokémon Red and Pokémon Blue, released in North America on September 28, 1998, both run on Japanese Blue's engine, Blue's redrawn Pokémon sprites and Blue's script. Only the version line-up was kept from the original pair: Western Red takes the wild Pokémon and exclusives of Japanese Red, and Western Blue takes those of Japanese Green.

So the Western Blue is a hybrid that never existed in Japan: Green's roster wearing Blue's improved presentation. Green itself never left Japan at retail, which is why a Japanese Green cartridge is the one first-generation box a Western collector's shelf is most likely to be missing.

The mascots tell the same story. Charizard fronts Red in both regions and Venusaur fronts Japanese Green, while Blastoise, the mascot Japan's special-edition Blue had already carried, fronts Western Blue.