Well, I'm not a recording artist, but I do occasionally put traditional words to new tunes - or more accurately, I try and tell the old story to a tune that I think communicates the story well. (So I'm adapting sets of words rather than simply transplanting them)

My reasons for doing so vary, but it's generally due to lack of suitable tune out there - either because I don't particularly "feel" the tunes I've heard for the song before, or if I'm using a source like a broadside (or something like the words jotted down from relatives by my mother), it might be for the very simple reason that the words didn't have a tune directly attached to them.

In short, there are lots of traditional songs I love the story or, or I like the message it's communicating - but for some reason none of the tunes feel interesting to me (and let's face it, there are a lot of bloody boring tunes out there), or natural to me as a way of communicating the song. In which case, I'd rather try singing the song a different way rather than simply drop it altogether.

Of course, there are also the various tunes 'created' through my misremembering the tune I first heard; this musical Chinese whispers can end up with my singing a very different melody to the 'source'. That kind of drift is another story altogether, but does not always have terrible consequences.