What was the reason for migration from the Dust Bowl?The Grapes of Wrath by John Steinbeck

Starvation was the main reason for the migration from the Great Plains area of the United States.  Because many of the farmers were sharecroppers, they had a limited amount of land on which to grow crops.  As a consequence, the precaution of rotating crops to prevent deprivation of minerals from the soil did not occur.  Added to this a phenomenal change in the temperatures of the Pacific Ocean as well as the Atlantic Ocean altered the Jet Stream and created "ideal" drought conditions, which led to the land drying up in areas such as Oklahoma where the Judds of Steinbeck's novel resided.  With no vegetation or trees to hold down the topsoil, the land became barren, and, when the wind blew, there were terrible dust storms.

Forced from their homes by starvation and lack of clean air, many of the Oakies traveled to California, especially the Salinas Valley where they had learned that there were jobs as vegetable and grape pickers.  This journey of the Oakies became  one out of the metaphoric desert of Oklahoma to the Promised Land of California.