This is a good question.
First, migrant workers had no place to call home. One of the conspicuous points about the book is that they were always traveling from one place to another. They moved to run away from trouble and to look for work. This is where we meet George and Lennie in the beginning of the book.
Second, because migrant workers were always on the move, they did not have any feeling of community. The only real friendship in the whole book was the friendship between Lennie and George. Everyone else was alone.
Third, migrant workers also had the feeling of being expendable. When Candy got old, he felt useless. When the men put his old dog down, it was symbolic. After the dog served its purpose, it was useful for anything. S
In short, migrant workers were poor, landless, rootless, and alone in a adverse world.