The reason that Japanese games frequently have a later release date is normally localization, it takes a lot of time to translate a game from Japanese to English, and even more time to then translate to the European languages. This used to be more of a problem before the Japanese software companies realised that their biggest market was outside Japan. These days generally only more obscure or text-heavy titles will have a long delay, the bigger software companies code all languages at the same time resulting in near parallel release dates. I think the most famous game to be released in US region before JP was Twilight Princess on the Wii which came out maybe a week earlier in the USA than Japan.