Why does Crooks tell Lennie that George isn't coming back?

It is not stated why Crooks does this.  However, if we read the text carefully, there are a few clues. 

Crooks is the low man on the farm.  He is a black man in the midst of white men who are racist.  We get a sense of this racism in Candy's retelling of what happened one Christmas.  The men watched Crooks fight a white man for the amusement of the men.  Also the boss yells...

It is not stated why Crooks does this.  However, if we read the text carefully, there are a few clues. 

Crooks is the low man on the farm.  He is a black man in the midst of white men who are racist.  We get a sense of this racism in Candy's retelling of what happened one Christmas.  The men watched Crooks fight a white man for the amusement of the men.  Also the boss yells at Crooks as an outlet for his anger. 

In light of these points, Crooks is beaten down emotionally and socially.  So, when there is someone who is arguably lower, Crooks takes advantage of him.  The one who is abused takes delight in abusing.  There is one sentence that makes this point.

Crooks’ face lighted with pleasure in his torture. “Nobody can’t tell what a guy’ll do,” he observed calmly. “Le’s say he wants to come back and can’t. S’pose he gets killed or hurt so he can’t come back.”

In this world, there is a twisted sense of pleasure.  Crooks is an example of this point.  Fortunately, Crooks stepped back when he saw Lennie's anger.