AsciiDoc gets a lot of things right, but it also has a lot of syntactic complexity and idiosyncrasies that were challenging for our documentation contributors. In documents with structural complexity, it becomes unwieldly very quickly. For example, having to vary the length of the delimiter line when nesting delimited blocks[1] is pretty arcane and leaves a lot of room for errors to creep in when reordering pieces of content. Even though AsciiDoc's extensibility model was compelling to us, it's really not designed for the kind of deeply-nested hierarchies that we needed to be able to express things like our integration builder[2] UI.
Anyhow, we have a section in our FAQ[3] that provides more context about why we didn't choose AsciiDoc. I remain a fan of AsciiDoc even though it didn't fully meet our needs, and Markdoc was definitely influenced by the things that we thought AsciiDoc got right.
[1]: https://docs.asciidoctor.org/asciidoc/latest/blocks/delimite...