The crucial difference is that static typing is a well understood problem.

The subset of "modern languages without null pointers" and "languages used to write a lot of serious software" is zero.

And notice "a lot" in this sentence. Majority of serious software is still written in languages like C++ or Java, which are either much less safe than Go or at about the same level of un-safeness.

While I'm sure someone wrote serious software in Haskell, very few (comparatively speaking) did. We can theorize why "modern languages without null pointers" failed to take off at the scale of Java or Python, but that they failed is a fact and that makes all their properties, including lack of null pointers, of questionable utility in the craft of actually writing useful software.

Go is a pragmatic, minimalistic languages, built on proven and well understood ideas.