It's less that but that the C tooling matured to the point where it was actually good enough to use. Particularly in terms of good enough output. Tight loops and often called functions were and sometimes still are (in SIMD terms) written in assembly so they could be hand optimized.
More exotic hardware also relied on the vendors providing a C compiler and the quality there could vary wildly.
More exotic hardware also relied on the vendors providing a C compiler and the quality there could vary wildly.