Probably because it would mean overhauling how world storage works for the first time in quite a while, and just generally be a fairly large undertaking. If that is indeed the reason, that does bode well for this suggestion seeing as Mojang's been overhauling the technical side of the game for a few updates now. If they are planning to do this at all, I'd expect it to come with the cave update, as you've said... hopefully that comes soon.