XML is only semi-structured/typed without schema. JSON and Mark are always typed.
Full formal schema definition, as in XML, is often a burden to ad-hoc scripting, which is common in JS. JSON/Mark provides sufficient type info for these adhoc usages.
JSON is not "fully typed". It just happens to have different syntax for strings, numbers, and booleans. But the application code still needs to come up with a way to distinguish between timestamps, enums, different object types, etc.
XML uses the same syntax for strings, integers, and booleans, but it has mature schema/typing tools that make it easy to apply more precise typing, which you'd want to do anyway to identify timestamps, enums, and different object types.
Full formal schema definition, as in XML, is often a burden to ad-hoc scripting, which is common in JS. JSON/Mark provides sufficient type info for these adhoc usages.