The answer to this question comes in Chapter 4 of the novel, after a thorough description of Crooks' room and possessions. When Lennie enters, Crooks is very abrupt and abrasive, telling him that he has no right to come into his room. He says:
"I ain't wanted in the bunk house, and you ain't wanted in my room."
When Lennie asks him why, he states the two reasons he feels why he is not accepted by the others on the ranch:
"'Cause I'm black. They play cards in there, but I can't play because I'm black. They say I stink."
Thus Crooks is excluded because of racism - the colour of his skin. This means, like other characters, he does not have a place to belong or to call home, contributing to belonging and loneliness as key themes of the novel.
Crooks is discriminated against because of his race. He attempts to explain this to Lennie, but he does not quite understand.