The alliteration is likely meant to subtly signal that George and Lennie are outsiders on the ranch, as they are the only ones without similar C-starting names. As the other answer mentions, the "C" is a hard sound, reflecting the harshness of the people who have been working on the ranch.

There are other reasons for the alliteration too. In a way, Curley's wife, Crooks, Candy, Curley, and Carson are all lonely people, alienated in their own ways. Curley's wife gets lonesome and is viewed with disdain by all the men on the ranch. Her husband treats her like an object. Curley is angry and insecure. Crooks is alienated because he is a black man in a prejudiced society. Candy is all alone save for his dog, which gets shot by Carlson, who hates the dog's smell. Like George and Lennie, all of these people dream of a better, more fulfilling life. And for all of them, as for George and Lennie, these dreams prove elusive.