there are still people bitching about things that don't matter like this? They have the same semantics and choosing between them is arbitrary. Json can be enforced with a schema but I doubt this is your concern.
JSON has a data model based on the lowest-common-denominator data structures available in programming languages, i.e. lists and dictionaries.
HTML/XML has a radically different data model that's based on documents that are filled with content with intermixed markup/metadata.
As a result, representing pure data is awkward in XML but straightforward in JSON, and likewise representing a document of mixed content is very very awkward in JSON but quite straightforward (and easily extensible) when represented as XML.
It's one of those "use the right tool for the job at hand" kind of things.
there is only one difference which is that the so called markup has a special syntax for "children". It's just some arbitrary design decision hardly a technical merit. As a transport they are both crappy as they are text based, why don't you complain about the fact that we waste a significant chunk of the Internet's traffic with these garbage, untyped transports.