Because it has a few fundamental quirks that make it a poor choice for enormous systems. It uses mutable data by default, the type system is not all that strong, and it has no facilities for making abstractions due to it being too simple. The developers of golang consciously throw out like 30 years of research in programming language theory because the idea of a language that gets "back to the basics" is appealing to a large group of (mostly good, experienced) programmers even though it's not a particularly good idea.

The first answer to this Quora question by one of the developers of D explains a few other points and also what golang is really good at:

https://www.quora.com/Which-language-has-the-brightest-futur...

I very much dislike that invalid states are left representable in idiomatic go error handling.