> XML was designed for documents; JSON for data structures.
JSON wasn't even designed for anything. It's literally the necessary and sufficient part of JavaScript that you could pass to an eval() to get a data structure out. It required zero tooling and even third-party module to hit the ground running.
As Douglas Crockford says: JSON was discovered, not invented.
The 'discovered not invented' framing is perfect. JSON's power came from already being everywhere - no parsing library, no schema debate, no namespace collision. It was JavaScript's object notation that happened to be useful. XML was a cathedral; JSON was a shed that everyone already had.