Crooks has nothing to contribute to a small subsistence farm. He is badly physically handicapped. George and Lennie already have one handicapped man who wants to join them, but Candy at least can contribute $350, which is over half the purchase price they need for the farm George has in mind. Crooks apparently has nothing, and this may be because he gets paid virtually nothing but room and board. He probably knows a lot about horses and mules--but there won't be any horses or mules on the farm George is thinking of buying. There would be no use for such animals because there are only two acres of land. They definitely want a cow, and a cow needs a lot of pasture land. They also want pigs, chickens, and rabbits. Steinbeck probably only involved Crooks in the dream to show that many agricultural workers would like a permanent home and some companionship, but there would be no feasible place for Crooks, regardless of his race. Steinbeck has Crooks change his mind about joining the other three men in their little commune, but it was Steinbeck's decision to exclude him.
Crooks is black, so while he is lonely like all the rest of the guys on the ranch, his loneliness is worse because of his skin color.
Crooks is not allowed in the white bunk house, and Curley's wife reminds him that he doesn't fit in. After she threatens to have him hung, he remembers his place in the world and that even in George, Lennie, and Candy's dream, he would still not be equal.