The simple answer is that Steinbeck was a writer, and he wrote what he knew, which was the 1930s California that he lived in. He wasn't too excited about this novella, as he thought it wasn't his best work. He would later say how continually surprised he was that the book sold so well, and received the acclaim that it did.
I think in the context of these things, we can get in trouble by trying to guess too much about his or any writer's motivations. That being said, I would argue that Steinbeck certainly wanted people to know the deplorable conditions of migrants and others during the Great Depression. The fact he wrote both Of Mice and Men and The Grapes of Wrath on that subject I think give us a clue as to what was on his mind. Of Mice and Men specifically gives the reader an insight to the most vulnerable in society at that time: blacks, women, the mentally impaired, the poor, the elderly and migrant workers.
To me, that's what makes it such high art. He shows us all of these people, distinctly and specifically, in the span of 108 pages.
The primary focus was explored above. I think that in writing what he knew from his own experiences, Steinbeck also helped to bring to light the plight of a group of individuals who were largely ignored and discarded as a result of the Great Depression. The plight of migrant workers was one that was treated with apathy and neglect. Their rootless existence, transient state of being, and victims to capitalism at its very worst was a narrative that went undetected until Steinbeck offered his views on it through his works. In articulating that which was silent, Steinbeck helped to explore and develop a larger vision of what it meant to be American.