Because of deep DEEP DEEEEEEEP xenophobia (which is a kind way of saying; the worst sort of racism that ever existed) over most of the populace which was not white or anglo of origin. As my father put it, It was sort of a mass hysteria which was gripping the nation. Heck, Life Magazine, in 1942 printed an article on how to tell Japanese from Chinese via facial features, skin color, bone structure, etc. It was like a page right out of Josef Mengele's handbook. Sad indeed. ~But I also understand that some German-Americans also suffered abuse on differing scales due to their heritage. Of course, America has learned from her past mistakes....