What does HTML have that XML doesn't have? Isn't XML a superset of HTML?
And a bunch of things called "browsers" that implement that spec. More or less.
I mean, come to that, aren't XML and HTML both just text? Isn't text, then, a hypertext? One can embed URLs in text, after all...
JSON as JSON isn't a hypertext in the same way that XML as XML isn't a hypertext. XHTML is a hypertext based on XML. (HTML itself isn't proper XML.)
There are hypertext specs based on JSON, such as: https://json-schema.org/draft-04/json-schema-hypermedia.html
But, without clients that treat the data as a uniform interface, it's all mostly pointless, which is why most JSON APIs stop at Level 2 of the Richardson Maturity Model, and why things like GraphQL are becoming more and more popular.
In the original model of the web (which is what Fielding was describing w/ REST) where HTML was being delivered to a browser, the uniform interface works: a browser has no idea what the HTML document means but can render it to a form that the human looking at it can make sense of.