The manner in which characters are referred in the novel could be Steinbeck's way of reflecting how little people know of one another during the time period. Steinbeck is writing of a time period where rootlessness and a sense of the transitory dominated American society. The individuals that dominate the setting of the narrative are "bindle carriers," individuals whose belongings are minimal and who go from place to place to make "their stake" and leave to another setting. Notice Steinbeck's description of the living quarters in the second chapter. The bare essentials are present because that is what represents the workers who are there. Most characters are known by a single name or a nickname because there is little in way of getting to know people in an indepth manner. It is a quick method of identification and is rooted in the present, the here and now. When someone is known by a first and last name, it represents a sense of the past as well as the present. Both names constitute an identity. The first name is part of the person's identity in the present tense, while the past is evident in the last name. In the world of Lennie and George, there is only the present, and single names reflect that. Curley's wife is known only as "Curley's wife," because it's a reflection of how she connects to the present tense of the workers. She has no identity outside of this. Slim represents "the best skinner" on the ranch, while Carlson and Whit represent the sense of the immediate. Even Candy's dog is only known as Candy's dog, and not by any other name. It makes Candy's guilt even more profound when he says that he should not have let Carlson kill his dog, a dog that did not even have a name. In this light, names are symbolic of the world that Steinbeck depicts, a world that Steinbeck believes is in massive need of transformation.
In Of Mice and Men, by John Steinbeck, with the story's overall message in mind about the disenfranchisement of so many during the Great Depression, I would assume that using surnames only compounds the sense of loss for those living in that era.
With the Great Crash of 1929, the economy literally fell apart while society and the government watched. Jobs were lost; entire fortunes disappeared when the banks failed; and, vast numbers of people lost their homes. Steinbeck writes about the travails of a family looking to find a new life in his novel The Grapes of Wrath. The experiences are much the same in terms of people moving across the country looking for a means to survive. The dream that George and Lennie share is to work hard enough to buy their own place and settle down, no longer needing to be forever on the move and working for someone else.
The use of surnames in this novel is just one more way that shows the demoralization of individuals. In this particular case, Crooks probably puts it most clearly. It has become the "American Dream," to stop moving and find a piece of land and settle down, with no need to travel anymore. Yet Crooks watches so many that come and so many that go, all harboring the same dream—but never realizing it.
I seen hundreds of men come by on the road an' on the ranches with their bindles on their back an' that same damn thing in heir heads. Hundreds of them. They come, an' they quit an' go on; an' every damn one of 'em's got a little piece of land in is head. An' never a God damn one of 'em ever gets it. Just like heaven. Ever'body wants a little piece of land...
To my way of thinking, people then weren't numbers as so often they are today: within society, we often lose our identity and become an identification number of some kind. However, during the Depression, people came and moved on so quickly, that people rarely identified with others as individuals, but saw only a blur of faces passing through. This loss of identity plagued individuals who had lost everything. Eleanor Roosevelt, wife to President F. D. Roosevelt, saw the need to establish a new sense of identity, which was lost during the Depression. The use of surnames only supports the theme of lost individuality.
[During the Great Depression], it was as important to the government to cultivate a unified American society as it was to return the country to economic stability. Eleanor Roosevelt brilliantly understood the need to develop a national sense of esteem and identity, and pressured her husband to take action.
Learning a last name supports the feeling of the loss of self as people never stayed long enough for workers to get to truly know one another. Based on Crook's observations, there was no point to trying to build a relationship with someone because there was no sense of community: just the constant change of faces.