As much as I like the ideas behind the semantic web, JSON-LD feels like the least friendly of all semantic markup options (compared to something like, say, microformats)
JSON-LD in a page's header is much easier for a CMS to present to the page author for editing. It can be a form in the editing UI. Wordpress et al have SEO plugins that make editing the JSON-LD data pretty straightforward.
But it's not a standard that is recognised, and so is no kind of metadata format.