They may be explainers of processes. Or the prose writers may be geniuses of characterization, like Tolstoy, Dickens, Dostoyevsky, and Balzac. When he talks about a birch tree, you feel your hands around its thickness. We poets just don't have the space to compete with prosers this way. De Maupassant and Flaubert were very good at this.
Some writers are fine at imagery, especially the imagery of the naturalistic world.
But even great prose writers also have glaring weaknesses. In no particular order of greatness, I present a number of remarkable and worthy prosers. Ernest Hemingway seems to me the most direct, truthful, and blunt pure writer of the 20th century. Hem ignored rhetoric and simply tried to write true sentences. Those who have read large amounts of him, however, know that he gradually became mannered, flat, stupid, incompetent, and befuddled.