There are llvm (and gcc) implementations of go but main/reference implementation was written from scratch in C and later moved to pure go. IIRC the authors wanted to achieve really fast compilation and a few other things that required them to start from scratch.
There was also the matter of figuring out llvm in order to use it. The initial Go compiler was based on the Plan 9 C compiler toolchain, which the designers of Go know well.
One of the other reasons relates to how stacks are handled for goroutines.
