Of Mice and Men is a novel in the genre of Realism, dealing with actual events and the sort of people who lived and worked through the Great Depression and the Dust Bowl of 1930s America. Steinbeck dismisses metaphor and allegory, instead telling the straightforward story of two men seeking a dream, and the harsh reality that keeps them from achieving it.
...two men emerged from the path and came into the opening by the green pool.
They had walked in single file down the path, and even in the open one stayed behind the other. Both were dressed in denim trousers and in denim coats with brass buttons. Both wore black, shapeless hats and both carried tight blanket rolls slung over their shoulders.
(Steinbeck, Of Mice and Men, Google Books)
Using short, simple description instead of ceaselessly comparing every attribute of the men to something else allows them a defined realistic quality, instead of the generic everyman that is common in fiction. The characters speak in era-appropriate vernacular, and talk about their own situation, not about world events or politics. Steinbeck was drawing on his own experiences as a migrant worker, and so knew that reality was far more interesting than anything fake he could make up. The feel and tone of the novel place it firmly in its era, and the novel's historical accuracy allow it to read as an overall representation of the exhausted people who traveled the country, looking for work.