Why did Fitzgerald chose Nick as the narrator when he is really not a central character to the tragic events that occur?

says, early on, that he
is "inclined to reserve all judgments" of other people.  He claims that
"Reserving judgments is a matter of infinite hope."  Perhaps if one can reserve
judgments, it shows that one always hopes to see the best in others.  In other words, Nick seems
to be willing to wait and see when it comes to the otherin the story:"represented
everything for which [Nick had] an unaffected scorn," but by reserving judgment, he found
that Gatsby possessed "an extraordinary gift for hope."  He tried to wait and see
aboutandtoo, despite Tom's overt racism and philandering and Daisy's snobbishness and
infidelity, and he eventually learned that they are "careless people [...] -- they smashed
up things and creatures and then retreated back into their money [...]."  He tried to wait
and see withtoo, though he recognizes early on that she is "incurably dishonest." 
Because Nick is willing to reserve judgments initially,...