Why doesn't Big Ma take Cassie's advice to move the farm wagon forward where the customers will notice it?

Big Ma doesn't take Cassie's advice because they are in a town dominated by white people. Big Ma knows that it is dangerous to draw attention..... to be perceived as undercutting "white" produce or overstepping their bounds. Ma is well aware that this is unfair, but she will do whatever is necessary to protect her family and land.