Have you really read the YAML spec? It's a rather complex grammar. I get that YAML is visually cleaner than JSON; however, unless we're constrained to very trivial use cases there's probably too much to think about. JSON's strong lack of TMTOWTDI is an asset here.
As for XML: namespaces, DTDs, entities, XSD, XSL, XPath, and on and on, have resulted in some absolutely horrendous architecture astronautism. XML itself is (mostly) perfectly fine but there are too many unqualified people in the industry wielding XML, poorly, and creating massive headaches. XML also presents some interesting security challenges.
