JSON-LD is just fantastic at bringing all the data in RDF to power real world JS/websites. Angular.js and JSON-LD plus SPARQL results in JSON is just so much fun to play with.
Seems like OWL never caught on, but doesn't OWL allow you to do quite a lot more useful reasoning? Whereas oftentimes people seem to be using RDF or its variants as a more complicated version of XML/ XML Schema.
I think XS2OWL implements XML Schemas in OWL. And OWL, like XML Schema, can be serialized into XML.
But I think the difference between XML Schema and OWL is more like the difference between high school algebra and calculus. You can probably explain both in terms of the other, and although calculus is probably more useful to explain a wider range of applications (via more complex maths that themselves are best explained with calculus), algebra will often do the trick. And, as any high school student can tell you, all you need is algebra because no one uses calculus in real life.
TL;DR: It is easy to write a JSON-LD bolt-on for an existing API. It is difficult to write a SPARQL extension function to create an API. Manu Sporny doesn't want people to conflate the issues.
