Looks like the code to bubble-sort the SYSINIT objects was committed a few months BEFORE this kernel qsort code was committed (late Aug 1995 vs early Nov 1995).
So that's why the sort-SYSINIT objects code didn't use qsort.
I'd also guess that there were fewer SYSINIT objects to sort back in 1995 than today.
see ~line 150 of sys/kern/init_main.c in https://github.com/freebsd/freebsd-src/commit/2b14f991e64ebe...