Asking why Nick is the perfect narrator for The Great Gatsby begs the question (i.e., requires you ask the question): Was Nick the perfect narrator?
Let's analyze a couple of Nick's traits as narrator and therefrom answer both questions. One trait Nick possesses is that, like he explains at the beginning, he is unusually tolerant. He realizes, through his father's help, that not all people have the same sense of human decency as others and that not all are able to develop a sense of decency to the extent of others: "a sense of the fundamental decencies is parcelled out unequally at birth." As a result, he is tolerant and non-judgemental of people's errors and foibles, of their lives.
A correlated and equally important trait is that while he has been sought out, he does not seek the confidences of "abnormal minds" of "wild, unknown men" wishing to bare their souls and have a confidant. Yet he is sought out because of his not overly common trait of being non-judgemental: "I'm inclined to reserve all judgments."
Another trait relates to his social, economic and cultural identity. He is of the middle class with parents who prepared themselves for profitable work and laid the way for Nick to engage in educational opportunities leading to profitable work. They are of the solid middle class without pretension to grandeur and equally without the struggling mentality of insufficiency. In addition, Nick is from the no-nonsense Mid-West with parents who are sensible and, based upon the comments he makes about his father, wise and extremely loving and accepting of Nick:
"Whenever you feel like criticizing any one," he told me, "just remember that all the people in this world haven't had the advantages that you've had."
He didn't say any more but we've always been unusually communicative in a reserved way, and I understood that he meant a great deal more than that.
A final trait to mention is that, because of his other traits, he is objective yet feeling so he is realistic in his opinions, particularly in his opinion about Gatsby--about whom no one agrees and about whom all had conjured wild fantasies--and even in his final opinion about Tom:
They were careless people, Tom and Daisy--they smashed up things and creatures and then retreated back into their money or their vast carelessness or whatever it was that kept them together, and let other people clean up the mess they had made. . . .
I shook hands with him; it seemed silly not to, for I felt suddenly as though I were talking to a child.
This analysis shows that Nick may in fact be the perfect narrator because while he provides the objectivity and the clear, sound opinions about strange and perplexing characters and events, he is a first-hand participant in the fateful events of Gatsby's last adventures and the final fulfillment of Gatsby's dream, the attainment (then loss) of the green light across the water. It is thus fair to say that Nick is a perfect narrator and that he is perfect for all his traits and for his objectivity and direct participation.