why does curleys wife treat crooks meanly

Crooks is relishing the company he has with Lennie. It's not the most stimulating conversation but it is company he so terribly misses. Curley's wife comes by on the pretence of looking for Curley (she is always "looking" for Curley) Crooks tells her to leave. Curley's wife turns vicious and threatens to accuse Crooks of rape. For Crooks, the punishment would mean certain death. Crooks is the only person that Curley's wife might have power over. His misery makes Curley's wife more despondent about her own misery. Humiliating and threatening Crooks gives her a little self-esteem.

Curley's wife is mean to Crooks just because her husband was in the whore house. She always loved to have the men attention but when that wasn't available and the black man who is consider to be in a less powerful postion than her is humiliting and telling her to get out of his room.