We can read the disabilities as being symbolic. Each disability is attached to a certain type of person and is representative of the idea that this person does not share equally in access to power and influence when compared to other people in the book.
Slim, George, Carlson and the boss share two important characteristics. They are (healthy) white (1) males (2). We cannot say this about Lennie, Candy, Curley's wife or Crooks.
Lennie stands in as the representative of the laborer who has no ability to improve his situation by means of tact, guile, or intelligence in general.
Lennie is a good worker and has the strength to do much of the farm work. Yet, handicapped by his lack of adult intelligence...
He is beholden to others for his safe-keeping and for his daily bread and is forced then to accept what others give him. Economically, this is true of George as well. Lennie's disability is, symbolically, identical with George's social disability, which is essentially defined by a lack of financial resources.
For the other characters with disabilities, we can see their lack of social power represented in their particular handicaps. Crooks and Candy are both associated with demographics that are excised from the circle of power and influence. Candy is old. Crooks is not white. In the examples of these characters we see the equation of physical disability with social powerlessness.
Candy is the old, disabled ranch hand who is helpless to stop the shooting of his dog and who knows that he too will be banished when he is no longer useful.
Candy's missing hand and Crooks' bad back are both symbols for the demographic, group-based prejudice that dominates in the world of this book. These men are both physically and figuratively powerless.