Nobody exactly knows why Psy's Gangham Style hasn't been that well received in Japanese charts, despite becoming n° 1 in views in Japan's Youtube. It is attributed to the fact that Psy never released a Japanese version of the song, a practice common among K-Pop stars to make it big in Japan.
From The Japan Daily Press:
"an unofficial manga made by a fan group, still to be released on December 31 shows a man bearing a striking similarity to PSY leading a Korean gang who kidnaps and then rapes a group of Japanese girls. PSY, with his “Gangnam Style”, transcended Internet and YouTube stardom to become one of the most popular stars of this year, and naturally his fellow Koreans are proud and protective of their fellow countryman and his image. Toss in an ongoing island dispute as well as some World War II wounds and you have an ugly mess in the making."