XML guarantees that tag precedes attributes which in turn precede contents, which is a great help when any sort of polymorphism is involved. Heck, anything beyond a tree of untyped arrays, maps, and primitive values adds ugly complexity to structuring and interpreting the JSON.

IMO much of that could be fixed with a JSON derivative that allows optional type identifiers before values. While you're there, guarantee support for comments and trailing commas, because despite design ideals, humans will write JSON manually, even using JSON for configuration files that are expected to be hand-modified!