If you came to me tomorrow and said "I want to build a language just like C but with non-contiguous stacks" I agree, I would use LLVM or GCC. But that's not what happened.

The history is three engineers decided to see if they could do better than C++ for what they did every day. That meant trying lots of things. One of the many was goroutines, but they needed a flexible platform on which to try lots of ideas that didn't make the final cut.

It just so happens, two of them had worked on a toolchain before. Ken's from Plan 9. (Which long predates the existence of LLVM.) And as he knew his compiler well, it was very easy to modify it to try these experiments.

In the end the language stabilized with several unusual features, several of which would be difficult to add to other compiler toolchains they were not familiar with. Is that the point they should switch to using LLVM?

Building on a toolchain you know that lets you experiment makes a lot of sense. Knowing a toolchain means you get to work quickly.

The end result still has useful features that LLVM does not. For example, running ./all.bash does three complete builds and runs all the tests. It takes about 60 seconds on my desktop. Last time I tried LLVM, it took minutes. Go programmers love fast compilers.