Because in Japan and USA, Pokemon is HUGE.
I'm doing this in Japan's point of view, because I think this applies more to Japan, rather than the USA. In Japan, Pokemon is like Mario. Everyone knows about it - it has been incredibly popular since the day of it's release. Like, click here to read about Pokemon's influence on Japan.
It may be incredibly popular in other countries, but you seriously can't beat Japan's Pokemon obsession. Also, I may have been imagining, but I think they named a Japanese city after Pikachu for a month. (I'm not sure if I read this or not, but whatever. xD)
Also, this is convenient for Nintendo as Nintendo is located in Japan, so it's very easy for them.
Hope I helped. :)