One reason would be massively reduced syntax overhead and better readability. I've seen plenty of XML files where XML syntax makes up more than 50% of the file's content, and trying to read the actual content is tedious. Now JSON isn't ideal either - technically you could get rid of all commas, colons, and the quotes around most keys - but I sure prefer `{"foo": "some \"stuff\""}` over something like `<foo><![CDATA[some <stuff>]]></foo>`
I agree, I would prefer JSON (or YAML) for example for configuration files. That is for stuff that humans actually read. I was thinking about using JSON/XML as a data exchange format between computers, because the context of this discussion has revolved about things like JSON/XML-Schema, JSON/XPath and SOAP/OpenAPI. There is a large trend to replace XML with JSON as data format for inter machine communication, and it is confusing to me.
