The use of an omniscient third-person narrator enables Chopin to tell a complete story that's not limited to the protagonist's point of view. This is key because the opening of the story begins with us readers knowing something Mrs. Mallard doesn't, and because the story ends after Mrs. Mallard has already died. If Mrs. Mallard were telling the story in first person, readers would be exposed to a whole different explanation of her weak heart, and the story would end very differently – and somewhat earlier.