The 1930s was during the Great Depression, and is the time frame that Steinbeck's Of Mice and Men is set in. Migrant workers traveled all over the country in those days picking and working in the various farms and orchards depending on what was being farmed at that time. So there was no concept of "home", and I'd say this was the first form of alienation. Wherever they were at, it was temporary, and you couldn't grow roots or make friends because you'd soon be moving on.
Always being on the move meant either dragging your family with you, which was difficult and expensive (so many families all worked together in the fields) or never being able to meet someone and settle down and get married. So you were isolated socially too, without a lot of hope in the near future of things looking up for you.
Lastly, the men on such farms were isolated from each other, whether through racial segregation, age differences or mutual suspicions and defensiveness. You lived alone even with others, and Steinbeck understood this very well when he wrote the novel.