Well, of course, Phil recorded his version a long, long time ago, and I'm pretty sure back in 1965, no record company would be too keen on putting out a 10 min plus version containing the complete lyrics.
Again, when you hear Phil's version - without prior knowledge of the poem - there is no sense that anything is "missing". The narrative is beautifully balanced and makes perfect sense.
And, of course, I'm pretty sure that if Phil hadn't done his version, chance are we would never have had any subsequent musical settings of the poem.
Mckennitt, one way or the other, would have been inspired to come up with her version from being aware of Phil's rendition.
McKennitt has a habit of not giving any credit. For example, she has never mentioned Nic Jones' version of Annachie Gordon as the inspiration for her recording of the song.