It's not very much less verbose, really. You still need a key and a value. The only thing you lose, really, is the end tag. For a complex text document in say TEI or Docbook, I don't see how this is much of an advantage.
In addition, arrays must be indicated clearly in JSON while you can consider children of an xml nodes to always form a list (be it empty or containing one element).
So while the two format are very similar, there is still some differences that doesn’t make them interchangeable for every usecase.
If there had been an opinionated SGML syntax which mapped directly and unambiguously to common language primitives and back, I'm sure it would have been more popular than JSON.