My understanding is that, even for the brand new x86-64 port, large parts of OpenVMS are still written in BLISS and MACRO, although I believe they prefer C for brand new code. A key component of the porting effort was modifying the existing BLISS and MACRO compilers to produce x86-64 binaries instead of Itanium ones. [0]
> This would have been 1991 or 1992 so DEC was probably still a BLISS shop. Maybe they didn't ship C manuals or something.
I suspect the actual reason – in 1993, DEC released a new C compiler for OpenVMS – DEC C, to replace their earlier VAX C compiler. [0] DEC C was designed to be a fully ANSI C compiler, VAX C was a pre-ANSI (K&R) compiler with various unusual proprietary extensions, along with some half-finished ANSI C compatibility tacked on the side. Probably, they were used to writing ANSI C on other platforms, and were given VAX C to use as a as a compiler, and were (quite understandably) frustrated by its inability to compile ANSI C code correctly.
[0] https://vmssoftware.com/docs/2021-04-27-x86-Compiler-Update....
[1] https://www.digiater.nl/openvms/freeware/v70/vmsfaq/vmsfaq_0...