English teaching at the Middle and High School levels shows a perhaps surprising propensity for literature that features the intellectually and/or developmentally impaired. Of Mice and Men is only one such novel. Ken Kesey's One Flew Over the Cuckoo's Nest, which is often assigned in high school English classes, is narrated by an American Indian man who is feigning near-catatonic stupidity. Similarly, Carson McCuller's classic novel, The Heart Is a Lonely Hunter, features a deaf-mute who is also described as imbecilic. Boo Radley, the mysterious shut-in in Harper Lee's To Kill a Mockingbird, is also a centrally important figure. And the widely taught short story (and subsequent novel) Flowers for Algernon by Daniel Keyes features a mentally retarded man who, due to miraculous medical treatment, is temporarily enhanced to a genius-level intellect. Several other classic novels that are less widely taught in schools, including Fyodor Dostoevsky's The Idiot, Graham Greene's A Burnt-Out Case, and William Faulkner's The Sound and the Fury, also feature mentally and physically challenged characters in central roles.
The question of why such a broad array of novels featuring the mentally impaired have proved so popular in the English curriculum is difficult to address briefly, but a few patterns seem clear. These novels are almost always quite sombre and pessimistic - exploring issues of social isolation, alienation and crisis - and these highly teachable, symbolically rich issues inevitably hinge on the mentally impaired individual in the novel. These characters serve as allegorical representations of a universal impairment that, the authors often suggest, is part of the human condition in general.
In Of Mice and Men, Lennie's extreme unsuitability for the social world provides Steinbeck with a means for exploring the needs and weaknesses of all his characters. For instance, Lennie is a foil for Curley's wife's confessions of unhappiness; he is a means for Candy's dream of replacing his dog with a new companionship; he tempers George's natural cynicism, giving his friend a sense of meaning and ambition for the future. In short, Lennie's extreme dependence on others represents every character's need for companionship. He is the symbol of alienation in general.
Similarly, in the other novels mentioned above, the central "idiot" figures provide a quick and concrete means of reflecting on the alienating cruelty of society in general. These characters are thus, almost inevitably, allegorical mutations of "man-in-general." Because the High School and College English curriculum so often prioritizes literature that highlights symbolism and universal-philosophical ponderings, it's quite easy to see why these novels, with their central symbol-characters, have proved so popular.
In recent years, a field in English Studies has emerged to challenge this treatment of the disabled as allegory: variously termed Disability or Dis/ability Studies. Scholars in this field often write about mentally and physically disabled characters as "marginalized individuals" - in other words, as human beings who have been systematically misunderstood and mistreated - rather than symbols of "man's isolation" or some other universal, allegorical interpretation.
Scholars in Disability Studies vary in their work, obviously, but many emphasize that the prevalent reading of the mentally and physically challenged as "universal symbols" is not necessarily inherent to the novels so much as it is to the teaching of the novels. For instance, one could easily read Of Mice and Men as a study of a "marginalized individual" rather than one of universal isolation as captured in Lennie. In general, such an approach encourages a consideration of the factors that lead a character like Lennie to end up in a situation of such ill fit. In other words, they replace the question that is so often asked of Lennie - why or why not is he unfit for society? - with a new question - what are the parameters of "social fitness" in the novel and how are they upheld?