Because people who know other programming languages, but not ML, might want to read your code, and it's nice if it looks familiar to them.
Maybe if Go didn't start as a hobby project inside Google, then they /might/ even have seen some opportunity in Ocaml / Standard ML, but now that Go exists, and has a quite good toolchain, the incentive is low. Go and its foundations are radically different from a FP language like OCaml though.