In ancient times there was no written language and every story was transferred orally. Of course, when you retell a story you change it in some way because you don't remember the exact wordings, it's only the meaning that matters. Hence after several generation the story could change in an unrecognizable way. Even if a person tries to remember the words themselves, the human memory isn't ideal, so there still will be some changes which may even render some text fragments meaningless.
It's different with poetry. In addition to the plot, here's one more factor which should be preserved on retelling: the rhyme. And with this additional restriction, it's in fact much harder to alter the story to a large extent. Maybe that's why a lot of epic literature is preserved in a poetic form? Just speculations.