That's an interesting question I hadn't considered. Off the top of my head, I might guess it's at least partly because I have the impression List and his followers were looking at the Runes more as "symbols" and less as "letters" - more concerned with their name and meaning, less with what sounds they represented in writing. And as you said, the lack of an intact Elder Futhark Rune Poem likely has something to do with it too.