Literature has many functions. Depending on which of these functions we honor and revere most, we might argue that Steinbeck both does and does not seem deserving of the Nobel Prize for literature.
If we focus on the belle lettres side of literature and look at fiction as a form of art that aims to create an expression of the human condition so poignant that it is beautiful, then Steinbeck wrote only one work that qualifies as a literary masterpiece - Of Mice and Men.
Many critics will say that Steinbeck's other work is marred, to greater and lesser degrees, by the presence of argument, polemic, and sentimenality.
If we consider the role of literature as a mode of social commentary and a vehicle for social change, then these criticisms become strengths. The arguments presented in The Pearland The Grapes of Wrath, among other works, become part of the Steinbeck's achievement in social commentary.
Winston Churchill won the Nobel Prize for literature also and after reading his tomes on WWII it is clear that he did not win for the belle lettres side of literature but for the social commentary side.
Given this dual context, we can argue in both ways - for and against Steinbeck's merits.