JSON-LD is a significant cognitive leap to reason about [1], and its descriptive power goes beyond what you could do with XML. A JSON-LD document is effectively an RDF document coupled with a JSON document, in a seemingly human-friendly form, although lots of people will recognize the letters but have no clue what they're reading.
For that matter, link relations too are a cognitive leap, and no one writes 'smart REST clients' anyway: these days, to consume a "REST API" you use the first-party official library, or something you found on github, or something you wrote in 3 hours where all the URLs are formed by string concatenation. Within one vendor's one particular REST API, the benefits of HATEOAS are typically minimal, which is why it's so frequently omitted and no one complains except REST pedants.
It also doesn't help that the field still hasn't settled down in ~10 years: JSON Schema, widely used for JSON schemas due to the lack of an official mechanism, recently decided they wanna get into hypermedia too [2], and there's also the enticingly named JSON API, which offers a similar data model too. And let's not forget about stuff ~2011-2013 like HAL [3], which didn't really become big, but never fully went away.
Swagger is absolutely much the new WSDL/SOAP ecosystem, complete with first-party code generators. In the WSDL days, all of your code generators were third-party, and due to enough knobs in WSDL and enough accumulated design baggage, it wasn't always interoperable [4]. But when it was, it was pretty magical for ~2003.
Swagger had clout and name recognition to kill the other schemes that are largely the same, but then they renamed it OpenAPI to play up the consensus against competing codegen/specs like RAML. Now that Mulesoft has agreed to bring RAML under the OpenAPI umbrella, there's less integrated competition, but then OpenAPI's uptake will be limited by those put off by serious tooling who opt for simpler description languages instead. It's a mess.
[1] https://www.w3.org/2013/dwbp/wiki/RDF_AND_JSON-LD_UseCases [2] http://json-schema.org/specification.html [3] http://stateless.co/hal_specification.html [4] https://blogs.msdn.microsoft.com/dotnetinterop/2005/01/17/ne...