One more advantage was that C and UNIX were designed for portability. They started on PDP-7 and spread onwards from there.
VMS was designed to sell mainframes so your choices were limited outside of that until the pedestal and workstation market arose around 88 (Alpha), MicroVAX, etc. Even then it was only DEC hardware. That's how you kill an OS.
There were already portable systems outside Bell Labs back then, and anyone that has written UNIX software knows how "portable" C actually was back then.
Had UNIX been sold with a price tag similar to VMS, without source available for universities to play around, and it would have been long dead by now.
VMS was designed to sell mainframes so your choices were limited outside of that until the pedestal and workstation market arose around 88 (Alpha), MicroVAX, etc. Even then it was only DEC hardware. That's how you kill an OS.