In an interview with game's producer, Koji Igarashi,found within the game's official Japanese strategy guide, he was asked why he set the game in Japan. He responded:

"Well, I know I keep saying that the protagonist of Aria of Sorrow, Soma Cruz is Japanese, but for the overseas version, we said he was a foreign exchange student living in Japan. That seemed like the best way for westerners to relate to the character, given that Castlevania is very popular in Japan and overseas. That’s why I asked the illustrator Aya Kojima to “draw him like a Japanese person who doesn’t look very Japanese.” And I’ve had people tell me he doesn’t look Japanese to them, so mission accomplished."
Contributed by ProtoSnake