MyST Markdown (the MD flavour, not the same-named Document Engine) was inspired by ReST. It was created to address the main pain-point of ReST for incoming users (it's not Markdown!).
As a project, the tooling to parse MyST Markdown was built on top of Sphinx, which primarily expects ReST as input. Now, I would not be surprised if most _new_ Sphinx users are using MyST Markdown (but I have no data there!)
Subsequently, the Jupyter Book project that built those tools has pivoted to building a new document engine that's better focused on the use-cases of our audience and leaning into modern tooling.
As a project, the tooling to parse MyST Markdown was built on top of Sphinx, which primarily expects ReST as input. Now, I would not be surprised if most _new_ Sphinx users are using MyST Markdown (but I have no data there!)
Subsequently, the Jupyter Book project that built those tools has pivoted to building a new document engine that's better focused on the use-cases of our audience and leaning into modern tooling.