Updating the RDF specification to create a new version is not an easy task. While getting new features added in is (relatively) not that hard, the cloud of related specifications, such as SPARQL, SHACL or OWL, is pretty large, and the whole ecosystem of applications and databases made to work with it is even larger. As such, getting new features actually out in the field is definitely not trivial, and some of the issues that may arise as part of that are still apparent today ‒ RDF 1.1 came out in 2014, while the latest version of SPARQL is still from 2013, and as a result, plain literals in SPARQL are still not quite exactly the same as string literals, even though RDF 1.1 doesn't make the distinction anymore.
A different approach is to develop a new version sideways, i.e. create a variant of RDF just with your own favourite new feature. This is similar to how XML is defined, as a set of specifications whereby the core XML specification, to the surprise of many, doesn't actually define anything related to namespaces, xml:lang or datatypes. The folks at RDF did a similar thing and created RDF-star, which is why this flew under my radar for so long.
