Throughout it's history, Japan has always been a spiritual hotspot of sorts, to the point that Japanese Spiritualists absorbed and transferred large portions of Shinigami culture to the japanese people as a whole during the Yamato period (this cultural leak began during the Kofun period, but saw the greatest overall acceptance during the Asuka period.)
Throughout the centuries, Hollow attacks and influential Spiritualists have helped to create a strong mythology for this land, and to shape it's people throughout time. Much of what Japan has become is a result of the effects these spiritual elements have had on the land and it's inhabitants. After the atomic bombs of World War Two, the land became a festering feeding ground, drawing in Hollows from all across Asia and the Pacific region.
As a result of that influx of Hollows, and the increased Reiatsu radiation that came with it, Japan's Spiritualist awakening rate skyrocketed, and ever since Japan has been the most 'spiritually rich' region on earth. So rich, in fact, that when the Hellgate was broken in 1986, it was into Hiroshima that the Living World end burst forth; spewing waves of demons hell-bent on vengeance into the living-world, only driven back by the valiant sacrifice of local Quincy and Spiritualists.