I love the short story format because just about any tale worth telling can be told well in 10,000 words or less. Sure, some sagas can be told in greater detail over thousands of pages, but the key components can always be collected and conveyed in a short story.
Even looking back to humanity before writing, oral histories were generally just verbal short stories. I'm not aware of any culture that passed down novel-length stories orally. For one thing, that wouldn't really be possible, and for another there just isn't any point. Even if a story started out that long it would get abridged and cut down quickly. The short story is the elemental structure of human culture.
Um, for some reason I'd thought that things like Homer's epic poems and the Elder Edda were really really long orally-transmitted poem/stories. (novel-length? maybe not the length of today's 1000 page monsters, but still book length) But I'm not a literature expert.
WH: Hm, I'd find that hard to believe, but I'll ask Megan.