It was actually mostly written in assembly, but used Pascal calling conventions and structure layouts since that was expected to be the primary language for application developers. As it had been for Lisa, as it was for “large” applications on Apple II, and as was the case for much of the rest of the microcomputer and minicomputer industry and even the nascent workstation industry (eg Apollo).

It was the Lisa system software that was mostly implemented in Pascal and some blamed this for its largeness and its performance. Compilers and linkers weren’t great back then; most compiler code generation was pretty rigid, and most linkers didn’t even coalesce identical string literals across compilation unit boundaries!

Lisa Workshop C introduced the “pascal” keyword for function declarations and definitions to indicate they used Pascal calling conventions, and otherwise followed Lisa Pascal structure layout rules, so as to minimize the overhead of interoperating with the OS. (I’m not sure whether it introduced the “\p” Pascal string literal convention too or if that came later with Stanford or THINK Lightspeed C.)