Japan has five more Kingdom Hearts games than the US
Although most of the Kingdom Hearts games have had worldwide releases, a handful of titles never made it overseas - Kingdom Hearts Final Mix, Kingdom Hearts coded, Kingdom Hearts II Final Mix, Kingdom Hearts Mobile, and the upcoming Kingdom Hearts Birth By Sleep Final Mix, giving Japan 13 releases to the US' five.
Granted, several of them are remakes and special editions, but they all contain some special weapons, brand-new cutscenes and hidden bosses that most gamers have never seen. If you want to see this stuff, you've got to import the games to get the full experience. Kingdom Hearts Mobile, the odd bunch of the group, is more an Internet hub than a game, where people can play mini-games, download game-specific content like ringtones, and snag limited-edition avatars.