Crooks, the black stable buck, is given his own separate housing because he is not white, like the other ranch hands. In other words, race is the central reason for his special living arrangement.
"Bitter and lonely, Crooks lives in isolation in the harness room" (eNotes).
This segregation takes on telling connotations in the context of the larger narrative around Crooks. In a social setting where the wealthy land owners live in a house and the proletarian, hired-hands live in a crude bunk house, Crooks is not the only one "put in his place." Rather, each character type is given a place of his or her own.
Consider Curley's wife. She is expected to stay in the house by herself all day because she is a woman and feels isolated there. Her resentment for the camaraderie of the men is often fully on display (especially in the scene that takes place in Crooks' room) and she feels so confined that she is willing to accept the ridicule and snide remarks from the men in order to find some companionship.
Crooks thus becomes one of several character types representing the many ways in which society can isolate people from one another for reasons that are no more than generic and categorical.
The shared dream that brings together George, Lennie, Candy and Crooks is a testament to the idea that each of these characters is limited and/or isolated in ways that make them similar. They yearn for the same things because, we might say, they suffer in similar ways.
"[T]he nature of the dream at the center of this story is specifically related to Steinbeck's critical understanding of a specific aspect of society in his contemporary California. The rootlessness and alienation which Steinbeck sees in the lives of California's migrant farm laborers are the real social conditions which he chooses to structure his story" (eNotes).
Crooks is given his own living quarters and alienated because he is "black" and the others are "white." But, importantly, he is not alone in his alienation, as it were. Gender, intellect, age and economic conditions also serve to alienate and isolate as we see with Curley's wife, with Lennie and with Candy.
Crooks is also stigmatised because of his crippled hand. He had caught it in some farm machinery a few years earlier and has since been delegated kitchen work, which is another source of humiliation for him since it is associated with a woman's work and domestic chores. Besides that, he has a bad back and is all bent over, from whence comes his nickname.
Crooks gives the other workers the "sour grapes" treatment, distaining them by being stand-offish and moody, but underneath (as he confides to Lenny) he is a very isolated and lonely man.
Crooks is physically set apart from the others because of his race; as an African-American, he is segregated from the white ranch hands, living in the harness room. He does not join the others in the bunkhouse, although George learns from the old man who shows him around that one exception to that rule was made the previous Christmas. Because it was Christmas, Crooks was allowed to "come in that night." Of Mice and Men was written during a time in the United States, long before the civil rights movement, when this kind of racial discrimination was the social norm.