Complex sentence: A large portion of the population was of Japanese descent (150,000 out of 400,000 people in 1937) and internment was deemed not practical, mostly for economic reasons.
Split into simpler sentences: A large portion of the population was of Japanese descent (150,000 out of 400,000 people in 1937). Due to the size of the Japanese-American population, internment was deemed not practical, mostly for economic reasons.