POSIX does not specify the init/rc script system, so it's not a factor here at all. A POSIX-compliant system could use Python scripts. macOS (which is UNIX 03 certified) uses launchd. A POSIX system has to ship the shell, not use it.
And FreeBSD isn't actually POSIX-certified anyway!
The real consideration here is simply that there are tons of existing rc scripts for BSDs, and switching them all would be a large task.
Probably never going to happen. There is a dearth of good scripting languages, and I would imagine any POSIX committee is like 98% greybeard naysayers who think 70s Unix was the pinnacle of computing.