Why not XML?

I suspect because the kernel developers do not want to be driven completely mad -- and struggling with the unreadability of XML, the even worse unreadability of XSD and the near unavailability of XSD validators that run at sane speeds and actually do what they're supposed to (let alone those that aren't written in Java and don't pull in half the universe)... well, this doesn't seem like a thing very likely to fly very far with people as generally interested in functional minimalism as kernel developers.