But I think, perhaps, that the real reason he never wrote a novel was that the form is largely dependent on character, and Borges had no real interest in, or facility for, the creation of psychologically vivid people. (Try relating Leopold Bloom orally in five minutes, or Mrs. Dalloway, or Anna Karenina. Their greatness as characters arises out of their irreducibility to the facts about themselves.) He wasn't much for fleshing out, and he was not the kind of writer whose characters ever had a chance of "taking over" from their creator.- From the New Yorker
More here on the underrated optionality of character in fiction.