"When comparing the markup formats for the purposes of kernel documentation, only the table support, which is much needed for the media subsystem documentation in particular, was clearly identified as being superior in AsciiDoc. Otherwise, the markup comparison was rather dispassionate; it really boiled down to the tools themselves...

... In the kernel tree, there are no tools written in Ruby, but there are plenty of tools written in Python. It was fairly easy to lean towards Sphinx in this regard."

As I read it, Asciidoctor is still superior, but their decision was to go with something written in Python that they could adapt for these specific needs.