why curley isnt treating his wife nicely

He is basically a nasty guy. He has a small guy complex and feels the need to look tough and unfeeling.

We never see Curly and his wife together. Curly's wife is perpetually "looking" for her husband. She is really just lonely. Curly too looks for his wife but only because he fears another man is with his posession. When she does open up she takes it for granted that everybody knows that Curly is abusive,

"think I’m gonna stay in that two by four house and listen to Curley.....I can’t talk to nobody but Curley"

We don't actually know if he doesn't treat her nicely, the only evidence of this is that she says to Lennie and I quote "I don't like Curley he ain't a nice fella"

They never actually meet in the book, they only meet when Curley is dead. We know that in the film he demands “shut up…get back in the house”. Curley wears the glove on his left hand to apparently keep it soft for his wife and that could possibly be his reason which could suggest that he makes her suffer for his satisfaction but still this isn't certain, personally I think Curley does have some interest in his wife but as we know he hasn't got very good social skills.