Well, I'm sure the creators of JSON weren't thinking of XML use-cases at the time. When it comes to solution-agnostic, self-describing, self-validating semantic markup, I'm not sure JSON-LD buys you much over RDF serialized to XML (for example).
You throw a JSON string at a simple parser and get an object back. That's why it won a place in my heart. Nothing prevented xml from being that except that's just not what the lib authors did. There was so much talk about XML being the One True Data Structure I think it ended up being a jack of all trades, master of none.
Take that away - plain unnamespaced tags, with no semantics beyond "this is a character sequence", common-sense business-specific tag names, and no schema - and you get something that's just as nice to work with as JSON is. And there are plenty of libraries (e.g. Jackson) that will serialize back and forth to objects. But too many people drank the semantic kool-aid and tried to autogenerate classes from schemata or other such silliness.