Our family homeschooled our 3 munchkins from K-12. As our kids got into the Jr. High--High School age range, one of the things I taught them about fiction literature is that regardless of whether it's fantasy (e.g. The Hobbit or The Chronicles of Narnia) or historical fiction (e.g. The Winds of War), the author created their own world, with rules and "natural" happenings of the author's choosing. Thus, Grapes is a world of Steinbeck's creation. It is supposedly true to the central US of Dust Bowl times. As noted above and in previous posts, it has significant deviations and/or inconsistencies from that reality - significant in the sense that those things are much of the core of the story and characters.
Like an architect or an engineer or a master carpenter, an author of fiction creates something, a whole world. In some degree, such human creativity mirrors in miniature The Creator. A master craftsman - architect, engineer, carpenter, author, whatever - loves and respects their craft and what they create. Or at least they should. I haven't read as widely as I might in late 19th or 20th Century fiction, but Steinbeck, in Grapes at least, is one of two writers (Thomas Hardy being the other, especially Jude the Obscure) I've found who seem not to love the worlds they created and peopled. The central characters, the Joad family, are somewhere between cardboard cut-outs and fully developed characters. And the lack of full development seems to be because they are set up to be knocked down. They are expendable pawns bearing Steibeck's message, not worth the effort of full and credible development. Love or hate Dickens, he developed and respected those who peopled his world: Oliver Twist and Mr. Bumble; David Copperfield and Uriah Heep; Mr. Micawber, Mr. Murdstone and Betsy Trotwood; Philip Pirrip and Mrs. Havisham. Even Upton Sinclair's Socialist tract, The Jungle showed greater respect for the family of his central character, Jurgis! Whatever Steinbeck's work was, before or after, Grapes is not a masterwork.
One of the supposed rules of fiction is that the author should show, not tell. Let the story sustain and communicate your message. In practical terms, what reader of a story wants the story interrupted by a lecture?! This is a, IMO, flaw that makes The Hunchback of Notre Dame marginally readable, and Les Miserables almost unreadable. Is the anti-war message of All Quiet on the Western Front any less clear for lack of a lecture? Is Dickens's condemnation of England's poor laws in Oliver Twist any less clear for the lack of a 20- or 40-page dissertation inveighing against the poor laws? Somewhere between 1/4 to 1/3 of Grapes is chapters explicating Steinbeck's views of what was happening in those parts of American society and history. Come on! Is Steinbeck too unskilled as a writer to weave his views into the story? Or does Steinbeck fear that his readers are too stupid to perceive and understand what he is saying through his story?!
In sum, in Grapes, Steinbeck doesn't respect history, the world he created or his readers. And I find that all the more frustrating, because, as I posted above, there are some truly excellent bits of prose throughout Grapes - the whole of which is much less than the sum of Grapes' parts.