Why does Conrad have one of Marlow's listeners relate the story, rather than make Marlow the narrator of the novel who speaks directly to the reader?

There is a sense of distance and objectivity that we get from the listeners which we would not get from Marlow. Marlow is too invested in the adventure and at times can become unreliable as a narrator. This point of view gives the reader a sense of objectivity.

In a sense in fact, although Marlow is not narrating, he is through the relating of his experiences to the listener narrator who is merely penning Marlow's story.