I think it's a bit misleading to say that "Google developed Go", as if it were some kind of strategic play. Go wasn't a top down product; it was created by the engineers who had the idea to create a new language, and hope to make it a popular one at Google. But even today, Go is not one of Google's top internal languages; Java and C++ apparently still dominate on the backend.

As for why Go isn't SML, just look at the original team. They had a background in languages like C (Ken Thompson , Oberon and Modula (Robert Griesemer) and Aleph, Newsqueak, and Limbo (Rob Pike). Nobody on the core Go team have ever been fans of functional languages or of complex type systems.