Crooks is a black man on the ranch among white men. For this reason, he is in a difficult place. He feels alienated more than the other men, who also experience alienation.
One of the main points of the novella is that the life of migrant workers is lonely. There is no idea of community and friendship, even among men who work and live together on the farm. This is why Slim is surprised to see that Lennie and George are actually friends. Slim says:
Slim looked through George and beyond him. “Ain’t many guys travel around together,” he mused. “I don’t know why. Maybe ever’body in the whole damn world is scared of each other.”
This is the context of the novella. All of this is far worse for Crooks. He is completely alone. At one point, Crooks says that no one ever visited his place. Moreover, he stays away from the other men. The pain that Crooks feels comes out in his conversation with Lennie. He says:
"You go on get outa my room. I ain’t wanted in the bunk house, and you ain’t wanted in my room."
“Why ain’t you wanted?” Lennie asked.
“’Cause I’m black. They play cards in there, but I can’t play because I’m black. They say I stink. Well, I tell you, you all of you stink to me.”
In short, Steinbeck paints a picture of near perfect alienation when it comes to Crooks, the lone black man on the ranch. He has it worse than the others.