Oral epics change with each singer
A sung epic is not a fixed text handed down unchanged. Each performer inherits it and then adds details and other elements as they see fit, so the version you hear belongs to one singer as much as to the tradition.
Politics and language limit fame
A great work can stay little known abroad for reasons that have nothing to do with its quality. An epic comparable to the most famous in the world can be kept in the shadows by the politics and the language of the place it comes from.