Realism
Steinbeck initially imagined this book to be a work of reportage, a first-person account of a London-like character who was a natural "boss" for a group of workers in the orchards. Steinbeck was particularly keen to keep bias out of the work: note that he's careful not to portray the workers as saintly or without fault.
His publishers convinced him to write this work as a novel instead, and the rest is history. All of this is to say that Steinbeck was committed to showing the life of migrant workers as it was, without much embellishment. Hence, he relied heavily on dialogue rather than long passages of narrative or commentary. Just keeping it 100, people.