When songs are passed on in an oral tradition, I would imagine that songs with clarity (or, perhaps, simplicity would be a better word) of language and image would be not only most appealing, but most easily remembered. They would invite the most repetition, and hence most versions. As a song is passed on, I would think that there would be a further polishing and stripping of non-essential or confusing language, either in attempts to "improve" the song or simply through the limitations of the singers memory.
But since a song is both melody and language, the simplicity or clarity of the melody would also contribute to a song's longevity and diffusion. Simple melodies are easier to retain, even for people without developed musical abilities and would most likely stay in common usage as lullabies and "hummers" or "whistlers". It would follow that melodies would also become more simplified and/or polished, as they are stripped of "non-essential" , i.e., difficult or forgettable notes.
Simplification of melody would lead to further truncation of text, as words are made to fit the "tune"; even to the point where images are blurred, non-sequitors occur and dead end rhymes retained.