In "By the Waters of Babylon," why did Benét select John to be the story’s narrator--why did he choose to relate the events through his eyes?

John is an ideal character for this story for several reasons.  One reason is that he is young, and it is the young who are usually more willing and anxious to find knowledge for themselves, and to break free from societal constraints in order to find that knowledge.  John believes in the rules and codes of his people, but because he is driven to learn, and young enough to be motivated, he is willing to stretch the rules a bit in his quest for information and wisdom.  It is only this willingness to cross boundaries that leads him into the place of the gods; his father, older and more steeped in follwing traditions exactly, probably would have never dared.  So, John's youth lends itself well to adventure and discovery despite restrictions.

Another reason John is a good choice is because he is, in his tribe, the son of a priest.  A priest has special privileges; going into dead places, touching metal, etc.  Also, a priest is required to go on a spirit walk, not like most of the other people there. It is is John's spirit walk that enables him to discover great things.  If he couldn't go where other men didn't, or didn't participate in a spirit walk, he wouldn't have learned so much.

John is another great choice because he is mature beyond his years, and is able to take his discoveries back to a wise father and decide, with him, what to do with the knowledge.  Also, as he journeys, we see things through a novice's eyes, through the eyes of someone who has never seen these things before.  It helps us to picture them ourselves, and see things from the viewpoint of someone coming upon them for the first time. John describes things so well, and has such maturity to draw wise conclusions from them; that helps us to understand them also.

I hope that helps; good luck!