I did most of my work on Unix before I started at Apple in '95. All of the new OS development was being done in C by then. I suspect that before most of the OS development had been done in 68K assembler, not Pascal. When the switch to PPC started, Apple needed a cross-platform systems programming language and Pascal was not it.
This article from '93 references how the industry mindset had switched to C/C++ and that pushed Apple.
One thing to remember is that at that time, both Macs and PCs were not very powerful machines and large applications were being developed for Unix workstations.