You got a very thorough and impressive answer already, so I just have one more thought to add. Steinbeck is concerned with the plight of migrant workers, to be sure; he also makes at least a passing commentary on race and gender in the characters of Crooks and Curley's wife.
Crooks is the only Black character in the work. He is a an angry, isolated, wounded, and frustrated ranch hand. He is resentful that he must live away from the others, yet he is confrontational to others when they invade his space. Once he hears about the dream of a farm, he is finally able to work up the nerve to ask for a simple request--to be able to come along and hoe the garden and be part of the dream. This is clearly representational of the plight of the Negroes at the time Steinback is writing.
Curley's wife is the only living female figure in the novella, and she never even gets a name--other than the epithets "tart" and "tramp" (and more). She was a stripper (or worse) who "married up"; she is grasping and seductive as she tries to keep herself occupied on the farm. Her last "targets" are the misfits, since the rest know her game and have no time or attention for her. She is cruel and unkind to those who are like her; they all have a dream--this woman with no name and no identity secretly yearns to be a famous movie star. Steinbeck clearly portrays her as a temptress, luring men to their demise; this is true for Lennie, only this time she is part of her own tragic ending. Despite the stereotypical persona he gives her, Curley's wife is more complicated than that. She is a tragic figure, trapped, isolated, and hurting, who lashes out at others in her despair and frustration. Her character, too, is Steinbeck's commentary, at least to some degree, on women.
John Steinbeck composed his novella Of Mice and Men--as well as in his seminal novel, The Grapes of Wrath- at a time when he was involved in California's social and economic problems. In Of Mice and Men, he writes about a group of people, the white migrant workers, who were to shortly disappear from American culture as farm owners began increasinly to employ low-wage foreign workers. During the 1930s there were thousands of these Americans who followed the harvestings in search of work; these "bundle-stiffs" are whom Steinbeck concerns his themes and characterization in Of Mice and Men.
Another threat to the American migrant workers--200,00 to 350,000 --was the employment of farm machinery, such as the combine used to harvest grain, which reduced the needed number of hands. In California, these "farms" were more like the timber industry or mining or oil industries. The owners were distant and rarely visited their land, hiring managers to oversee the farming, so there was a depersonalization that existed. Profit was the only concern of these owners.
It is this alienation of the "bundle-stiffs" about which Steinbeck, who was a Socialist, concerns his narrative; these displaced men have no friends, no home as there are forced to leave family and friends in search of work during the Great Depression. In this theme of alienation, Steinbeck includes no women other than the temptress Curley's wife, who acts as another of the forces which spoil to the fraternity of the men.
As a novella, Steinbeck's Of Mice and Men permits only room for his main theme which concerns the American migrant worker. Although he is concerned about him, Steinbeck later became involved with the plight of Mexicans after the "Zoot-Suit Affair." His treatment of Crooks in the novella is realistic, and he uses this character to underscore the terrible aloneness of all men during the time of the setting. Since women were not a real part of the itinerant work force they simply do not figure into Steinbeck's theme.