Earlier quoted context omitted.
The success of UNIX is inseparable from C and vice versa. C made UNIX one of the first portable operating systems and, by virtue of being low-level, provided performance.
The same could be said of OSes using Algol and Mesa. They just hadn't successful startups using them.
Sun used Unix because Sun was co-founded by Bill Joy. By that time, Joy was already deeply involved in Unix (per BSD). If he didn't find Unix and C likeable and up to the task, they would have made different choices.
SGI and Sun were just one of many catalyzers, like a lot of programming languages have catalyzers.