i would define an art song as one that is primed for performance; these are usually the memorable ones. There are many versions of folk songs but the one that is most memorable is the one associated with a particular singer. It's that variant that is significant.
Besides not all variants of a particular song are equal. Some are more appealing.
I think this is why certain songs continue and others are forgotten.
not to labor the point, the performance often makes the song happen and this is what the listener remembers more than the actual song itself unless there is an academic interest in the song which sort of works against the emotional effect receiving the performance. When we hear a song, we are hearing an interpretation of that song which relies on the skill of the performer as much or if not more than the song itself.