The first PDP-11 Unix was written in assembly, and was thus a from-scratch rewrite of PDP-7 Unix (also in assembly, for a very different machine). The rewrite into an early version of C came later, done initially for the same reasons you'd prefer C to assembly in, say embedded systems (maintainability) -- and the idea of using the C code as the basis for a port came a few years after that.

See https://en.wikipedia.org/wiki/History_of_Unix