In chapter 4 in Of Mice and Men, why is the stable buck given his own room?

The above editor is correct:  the information you need is
in chapter four of the novel, rather than in chapter
two. 

If you need evidence for your answer, it begins on
page 75 of my edition.  The entire chapter takes place in Crooks' room, which is
adjacent to the barn and is entered through the barn.  When Lennie appears and explains
that he saw Crooks' light on, so he came in, Crooks
says:

"Well,
I got a right to have a light.  You go on get outta my room.  I ain't wanted in the bunk
house, and you ain't wanted in my room."

"Why ain't you
wanted?"  Lennie asked.

"'Cause I'm black.  They play cards
in there, but I can't play because I'm black.  They say I stink.  Well, I tell you, you
all of you stink to
me."

Crooks isn't allowed in
the bunk house, because of segregation.  The races are kept apart.  The reader assumes
that the idea of Crooks stinking is a stereotypical idea the whites project upon him due
to their prejudices.  They don't want him in the bunk house because he is
black.