I think a big reason for lack of popularity is familiarity of graphs in general. Most people wouldn't know of a good graph editor I would guess. Everyone knows documents and spreadsheets/tables/lists/folders, but it's rare to see a graph anywhere. Online and offline. If I have to think about an example of a graph in real-life what comes to mind is: a corkboard with strings going between them to track a criminal in a tv show, or a subway network map. In VSCode/IntelliJ, there is not one graph I look at frequently, even though pretty much all code is a data flow and dependency graph.
I think part of this is that graphs always appear like a complicated mess, and we prefer hierarchies and categories.
I would really like a tool like Airtable for graphs. You start with spreadsheets with columns relating to other columns, and then you view the graph next to it as you go. I don't know of a popular tool that does this. It's funny because behind-the-scenes of spreadsheets there is always big dependency graph that updates cells as changes come through.
All the specs feel overly-complex too. Like a relic from the XML/SOAP days. For such a simple base concept (subject-predicate-object / entity-attribute-value) it feels like overkill. It's interesting though thinking about how JSON won, while being extremely inferior to XML. Although I think this ability to move fast has left us with a ton of untyped data lying around, and plenty of ad-hoc data transformations.
I'm interested to read into the EasierRDF doc you sent - looks very interesting.