Why is Curley's wife so lonely on the ranch in Of Mice and Men?

The primary reason why Curley's wife is so lonely is due to the fact that she is the only female on the ranch; she has no one to identify with, does not have a sympathetic husband, and lacks social interaction. Unlike the workers, who are all male and relatively enjoy each other's company in the bunk house, Curley's wife lives an isolated life and struggles to have social interactions with the men. The majority of workers on the ranch view her as a threat and think that she is a "tart" because she is constantly flirting with them.
When one views Curley's wife's behavior from her perspective, she may simply be attempting to have a conversation with the workers because she lacks opportunities to socially interact with others. In addition to her isolated position as the only female on the ranch, Curley's wife is also lonely because her authoritative husband forbids her from interacting with anyone else. Curley is an extremely insecure man, who does not trust his wife and is an abrasive, callous individual. Curley's refusal to sympathize with his wife and allow her to socialize with the workers also contributes to his wife's loneliness.

Curley's wife is lonely because there are no other women around to whom she can relate, and the ranch hands avoid any entanglement with her because she is the wife of the boss's son. She also isolates herself by cruel remarks to others.
Isolated on the large ranch, Curley's wife--who is but a genitive of her husband in this novella as she is given no name--hungers for attention and communication with others, although she often seeks this attention in the wrong manner. For, knowing that she is the only woman and that she is attractive, Curley's wife positions herself in a seductive manner:

A girl was standing there looking in. She had full, rouged lips and wide spaced eyes, heavily made up....She put her hands behind her back and leaned against the door frame so that her body was thrown forward. (Ch. 2)

But, even though she physically entices the men, they are afraid of tangling with Curley, who is very pugnacious, and can get them fired. Consequently, they do not respond to her seductive looks, nor do they engage in conversation with his wife. To her dismay, they instead leave her alone.
Curley's wife is also lonely because, although she desires company, she has the tendency to exploit the weaknesses of men with whom she has contact. For instance, she exploits the vulnerability of Crooks as she reminds him that he is a social inferior and outcast, rather than being friendly to him. So he retreats and will speak to her no more.