Why did Tina's parents live away from their daughters for two years?

As a result of World War II, a large diffusion plant was being constructed in Nashville creating an urgent need for laborers and other workers. Tina's parents, Zelma and Richard, travel to Nashville to seek employment at the plant, leaving Alline with their maternal grandmother, Mama Georgie, and Tina with their paternal grandmother, Mama Roxanna. Zelma and Richard live in Knoxville for two years, away from their daughters.