It's easier to work out the vocal range of a song from notation. Flip through a whole lot of books and note which ones look okay.

Maybe the melodic shape makes a difference? There seems to be a cultural universal that the oldest songs in any culture are those that start high and descend in scale-like patterns (like "The Trees They Do Grow High"). This is easier for most people than leaping up and down.

There are very few traditional songs that go beyond an octave and a half. "Danny Boy" is one of the rare exceptions.