> It would be nice to hear about the rationale from a primary source.

Assembly language was deemed a last resort, especially as portability
was coming to the fore. As I wrote in A Research Unix Reader,
"Assembly language and magic constants gradually declined from the
status of the 'real truth' (v4) to utterly forgotten (v8)." In v7,
assembler usage was demoted to the bottom of syscall man pages. It
could also be found in /usr/src/libc/sys/*.s