Why does Crooks say that George, Lennie, and Candy will never attain their dream of owning their own land?

Crooks is the black stable buck in Steinbeck's novella Of Mice and Men. Because of racism and segregation he lives by himself in a room in the barn. In chapter four most of the workers, including George, have gone into Soledad, leaving Lennie, Candy and Crooks behind. Lennie is in the barn playing with his puppy when he sees Crooks's light. He stands at the doorway, and despite initial protests, Crooks invites Lennie into the room. Lennie, as he often does, is soon talking about the rabbits he will take care of when he and George get their own place. Crooks scoffs at the idea that a worker could ever get his own land. He tells Lennie,

"Ever’body wants a little piece of lan’. I read plenty of books out here. Nobody never gets to heaven, and nobody gets no land. It’s just in their head. They’re all the time talkin’ about it, but it’s jus’ in their head.”

A little later when Candy, who is looking for Lennie, comes into Crooks's room he too announces that the three men will soon be getting their own place. When Candy tells Lennie they can make money on rabbits, Crooks again repeats his criticism:

“You guys is just kiddin’ yourself. You’ll talk about it a hell of a lot, but you won’t get no land. You’ll be a swamper here till they take you out in a box. Hell, I seen too many guys. Lennie here’ll quit an’ be on the road in two, three weeks. Seems like ever’ guy got land in his head.” 

Candy then claims that they even have the money, which he is contributing and the dream will soon be realized. Crooks is still disbelieving and questions the whereabouts of George. He says,

“An’ where’s George now? In town in a whorehouse. That’s where your money’s goin’. Jesus, I seen it happen too many times. I seen too many guys with land in their head. They never get none under their hand.” 

Eventually, however, Crooks is convinced that the dream is a possibility and even offers to join the men. He says,

“ . . . . If you . . . . guys would want a hand to work for nothing—just his keep, why I’d come an’ lend a hand. I ain’t so crippled I can’t work like a son-of-a- bitch if I want to.”

He is then appropriately interrupted by Curley's wife who is, of course, looking for Curley. She lashes out at Crooks when he asks her to leave and he ultimately lies to Candy about not wanting to join them. Not only is Crooks's dream destroyed by Curley's wife, but the other men's dream as well, as she is the cause of the tragedy which follows in chapter five and six.