Will, I wonder whether the reason is that with tunes the folk process is still very much alive and can be observed in action (as the examples of "The Sweetness of Mary" or "The Flying Cloud" illustrate). Also, tune players seem more relaxed with the idea that the notes are merely the framework of a tune for the musicians to work around - many experienced musicians won't play the tune exactly the same way twice (in English tune sessions, anyway - some Irish sessions seem to insist on more accuracy).

That seems to be how traditional singers treated songs, with both tune and words likely to vary from one repetition to the next. It's something which revival singers mostly appear to have lost (just look at the number of complaints about singers starting a version of a well-known song and the audience or accompanying musicians taking over with the "proper" version).

In that sense the "tradition" is perhaps more alive today with tunes rather than songs, where fixed versions of songs seem to have taken root.