John Steinbeck grew up in the Salinas Valley and knew the setting and the kind of men he was writing about. However, Of Mice and Men was an early effort and Steinbeck was not a prominent writer at the time. He would have been described as a "regional writer," and his bucolic subject matter characterized him as such. California had a small population and was far distant from the centers of wealth and power in the East. He was seriously planning to turn Of Mice and Men into a stage play, as is described in the eNotes Study Guide Introduction. This explains why his novella is so short and why there is so much dialogue. The play was probably more important to Steinbeck than the book, because the play, if successful, would reach the important people in New York City and on the East Coast. It would also make more money than a short novel by an unknown writer.
Because of its heavy reliance on dialogue, the book was easy to adapt to a stage play. The settings in the book are minimalistic. The main sets are a bunkhouse and a barn. The setting by the riverbank is beautifully described, but it could be represented on a stage by just a couple of bed rolls and a "campfire" with a red lightbulb under a few sticks. It is noteworthy that there are virtually no outdoor scenes in the book except for the opening and closing scene at the riverbank. Big crews of men work outdoors on the vast California fields with big teams of horses, but none of this is described in the book because it would not be possible to show it on a stage. Even when the men pitch horseshoes, all that is described is the occasional clang when a horseshoe hits the metal post.
It seems likely that Steinbeck decided to make one of his principal characters feeble-minded because that would enable him to have George explain everything to Lennie that the author wished to have explained to the reader and to the theater audience. Lennie not only has trouble understanding but also has trouble remembering, so George can repeat important expository information, such as where they are going, where they are coming from, how Lennie is supposed to behave, and where Lennie is supposed to meet George if he gets in trouble and has to run away (as he did in Weed). Steinbeck had to invent some rather implausible business with Aunt Clara to explain why George is taking so much trouble to serve as Lennie's guardian.
Steinbeck's heavy reliance on dialogue, especially between George and Lennie, allows him to have George, with Lennie's occasional interjections, spell out their so-called "American dream" of owning a little piece of land. This dream is the motivation that drives the story. Otherwise there would be very little in the way of a plot. We empathize with George and Lennie because of their dream, and we share in their tragedy because the mishap in the barn makes fulfillment of the dream impossible.
Steinbeck's strength as an author is based heavily on his sympathy for the underprivileged and politically impotent. He writes with strong feeling and communicates his feeling to the reader.
Steinbeck chose to write about the little people of the world because of his philosophy and liberal political beliefs. He wanted to show that even the humblest men are complex creatures with hopes, dreams, desires, problems, and personal relationships. This is characteristic of all of his early writings. He suggests that many of his characters have talents that would offer them better lives if only they had education and opportunities. He feels that the social system is unjust because it denies such opportunities to a large portion of the population. In his later novel The Grapes of Wrath he makes it clear that he believes the government should do more to offer help and protection to the underprivileged and hints that the voters should see to it that their representatives take affirmative action.
It should be noted that America's entrance into World War II after the bombing of Pearl Harbor in December of 1941 brought about great changes because of the enormous demand for soldiers and sailors, as well as the need for workers in ship building, munitions manufacturing, aircraft production, and many other areas of the great war effort. Furthermore, there was an increasing mechanization of agriculture. The teams of horses described in Of Mice and Men no longer exist, because one powerful tractor can pull more than a hundred horses. There is also less demand for human labor in many aspects of agricultural production because of the invention of all kinds of specialized machinery.