Why do we not truly get to know Crooks until more than halfway through the book Of Mice and Men?

Crooks is introduced in chapter two of the book. Candy says he's a black man (the N word is used throughout) who works in the stable:

“Yeah. Nice fella too. Got a crooked back where a horse kicked him. The boss gives him hell when he’s mad. But the stable buck don’t give a damn about that. He reads a lot. Got books in his room.” 

He is also mentioned in chapter three. Steinbeck writes:

The door opened quietly and the stable buck put in his head; a lean negro head, lined with pain, the eyes patient. 

Both of these descriptions reveal things about Crooks that are further explained later. He becomes a very round character in chapter four when Lennie comes into his room in the barn. In this chapter Steinbeck expounds on the theme of loneliness which has only been hinted at in earlier chapters. In chapters one through three, Steinbeck concentrates on his other two major themes: the importance of friendship and the American dream, so this may be why a full description of Crooks doesn't come until more than halfway into the book. It is in this chapter and chapter five that both Crooks and Curley's wife are looked at in more detail. Both characters are painfully lonesome. Crooks says,

"S’pose you didn’t have nobody. S’pose you couldn’t go into the bunk house and play rummy ‘cause you was black. How’d you like that? S’pose you had to sit out here an’ read books. Sure you could play horseshoes till it got dark, but then you got to read books. Books ain’t no good. A guy needs somebody—to be near him.” 

Lennie is used as a sounding board by both Crooks and Curley's wife. Lennie is considered a safe person to talk to. He simply listens without passing judgement. Crooks and Curley's wife would probably not reveal such intimate thoughts to other characters because they would be afraid of being judged in a derisive way. Most of the workers live solitary lives without ever connecting with anyone else. Slim says it best in chapter three:

"Hardly none of the guys ever travel together. I hardly never seen two guys travel together. You know how the hands are, they just come in and get their bunk and work a month, and then they quit and go out alone. Never seem to give a damn about nobody."

For a time Crooks even becomes part of the dream of buying the "little piece of land" until Curley's wife threatens him and he returns to his quiet and "aloof" demeanor. It is ironic that Curley's wife, who is also a victim in this story, should be the one to shatter Crooks's dreams.