Many japanese developers target the Japanese audience with games like gundam, onimusha, naruto, genji, etc....
Sure, they release it in NA and Europe but they don't sell as good as in Japan.
Because xbox360 doesn't sell well in Japan, there's no reason to port a game if not enough people will buy it. Porting a game is cheaper than making a brand new game but that doesn't mean it will automatically result in profit.
Also, 'human resources' are limited. The same guys that are working on the xbox360 version of a PS3 game could instead be instructed to work on a new PS3 game which will result in more profit.
u have forgotten that people in north america and europe buys japanese games, right???