I'm not 100% sure, but I believe its because during that time the US was fighting Japan and many American citizens were fearful that Japanese Americans would turn against them in support of Japan. So for the safety of the Japanese Americans and for the overall comfort of the other citizens, the U.S. government at the time felt it was best to place them in internment camps. What the U.S. government didn't realize was that this caused more problems and cost more money in the future. I hope this helps.
