Okies – the owners hated them because the owners knew they were soft and the Okies strong, that they were fed and the Okies hungry; and perhaps the owners had heard from their grandfathers how easy it is to steal land from a soft man if you are fierce and hungry and armed.

– John Steinbeck

The Grapes of Wrath, Chapter 19. The landowners in the West are prejudiced against the Okies because they are strong and hungry for land. The owners remember their own history, when their ancestors had the same hunger and stole the land from the Mexicans. So they fear and hate the Okies.