Russ Cox gave a really good answer to this question. I have to paraphrase because I can't find the original comment. It was done for implementation speed. They

Russ Cox gave a really good answer to this question. I have to paraphrase because I can't find the original comment. It was done for implementation speed. They had a compiler backend which they, the early Go team, were all familiar with. It was small so they could quickly change it to suit Go, rather than the other way around. Russ Cox expressed the sentiment that if they had tried to use LLVM there would have been a lot of up front work that would have taken a long time and the Go project probably would have fizzled out. Using the plan 9 tool chain they were up and running very quickly allowing them to focus on the most important work, designing the language itself. (Probably shouldn't attribute this to Russ Cox, I've probably read into his (rather briefer than what I wrote above) comments) In my opinion this was the right choice. If LLVM turns out to be the best backend for Go it's easy to use it later. However, the language itself is largely fixed after 1.0. Better to focus on the language than the compiler backend which can be swapped out later.