What is the most likely reason Hemingway wrote The Old Man and the Sea using an omniscient narrator?
Hemingway writes in a plain, tough style which is best suited for 3rd person omniscient narration. This is the style of great religious prose. Of parables. Of the Bible. Of journalism. It guarantees the most objectivity. The most ethos (ethical argument). The least pathos (emotional argument).
Notice: I'm trying to style this answer using it.
Hemingway wants Santiago to be a Christ figure. If he would have made him the narrator, his suffering might have come across as self-absorbed. It would have tipped his hand toward the reader feeling too much sympathy for Santiago.
This style is usually "I" oriented, but Hemingway allows the reader to get inside Santiago's head by having Santiago speak his thoughts. Like a soliloquy. He talks to the fish and the boy, not present. His speech then becomes apostrophe. Even though the plain/tough style allows for a detached narrator, like Merusault in The Stranger, Hemingway opts for the best of both worlds: 3rd person narration with lots of 1st person soliloquy. Since he's an old man on a boat in the vast ocean, soliloquy works.
Below are characteristics of the plain/tough style of narration for which Hemingway is known: high frequency, monosyllabic words, contractions, articles, 1st person pronouns, action verbs, active tense, Anglo-Saxon words, simple sentences, short, choppy and compound sentences (lots of coordinating conjunctions “and”), 1st Person (I –oriented), subjective, informal (causal), male (macho), ethos (credibility), trustworthiness of the writer or speaker, inductive reasoning, stream of consciousness, in medias res, irony, metaphor, simile, allegories, double meanings, journalistic.