I think Uncle DaveO has a good point about an instrument being 'cool'...Different instruments are also ranked in order of street cred. But that is also the case with some adult musicians, lol! I have heard several capable and talented vocalists be asked why they don't play an instrument as well as sing..."but surely you could learn at least a few chords on the guitar?" I think many still consider the voice a 'lesser' instrument' than others. I don't agree with that view, myself.
Greg, good to hear you are encouraging singing in schools. Did you try to get the oldest children in primary school to sing on their own as well as in a group? I find those who are used to it, are fine singing in small groups or individually but when I work with children who have had little experience at singing like that, it takes a lot more encouragement.