Answer:
The years since 1929 were generally prosperous ones for the urban Northwest, moreover, but rural parts of the region seldom benefited as much.
The war itself jump-started the demographic changes, as is evident in the changes between 1940 and 1950, and other events kept them going.
Washington consistently possessed a larger and more diverse population than Oregon or Idaho.
Immigrants began arriving from eastern and southern Europe, as well as other continents and regions like Asia and Latin America. This led to increased diversity in the United States.
The United States began an economic boom that brought unparalleled prosperity to a majority of its citizens and raised Americans expectations, breeding a belief that most economic and social problems could be solved.