For starters, we would know nothing of Max's family background. From Max we learn of his childhood trauma: the mysterious death of his mother and the equally mysterious--at least at first--imprisonment of his father. In a practical sense, the language would be elevated because of his almost miraculous vocabulary talent. We also are taken in by the simplicity of Max's personality, by his desire to learn, by his frustration with his own weaknesses. We can relate to him. It is much more difficult to relate to Kevin on such a level--Kevin, who is physically impaired yet mentally strong. Lastly, if Kevin is the narrator, we would be left without an explanation of Max's destiny at the end of the book after Kevin's demise.