Why do gaijin (foreign) guys who could never score at home always link up in Japan?
Popular theories usually mention how sad the average Japanese salaryman is and the gaijin boyfriend being just a fashion accessory, free English conversation partner or novelty item for a Japanese girl to impress her friends with. Other possibilities include the Japanese girlfriend feeling freer due to not being restricted by the Japanese language (no way to say I love you in normal Japanese conversation!) and his family being several thousand miles away.
I believe the main reason is that the gaijin male quickly convinces himself that he can throw off his lack of success at home, and so he does- i.e. purely the power of postive thinking and self-belief. Any unusual social skills that still hang around from his old life can also be passed off as being a foreigner or other cultural or language misunderstandings.
There is also the general question of why outsiders instantly become better dating prospects in almost every society, which is possibly tied into the desireability of bringing other genes into the limited gene pools that the often isolated groups of early humans would have been.
More on Gaijin on the Gaijin explained page.