Of course it could be done. It was done... with millions of lines of extra code and complexity.

I'd say the immediate thing that comes to mind is that there isn't a one-line-code that would deserialize an XML message to language-universal collections (arrays/lists/vectors, dictionaries/hashmaps/assoc. arrays). The closest was XStream in java-land, which I used heavily before JSON became widespread.

Secondly, it doesn't have a natural specifier for THIS IS A LIST. You're guessing or relying on mappers/marshalling to produce such things, while JSON can naturally map from JSON to an object graph without excessive mappers, annotations, etc.

Other complaints: The attribute/tag dual-duty was also problematic, confused the entire problem space.