> how come so many successful Systems programs (e.g. Kubernetes) are written in Go

This is because what you call "systems" is not the "systems" that the lack of GC would benefit from. For all intents and purposes Kubernetes could be written in Python or any other general purpose programming language (and for example OpenStack is in fact written in Python).

On the other hand, systems like "writing an operating system kernel" or "running on a microcontroller" are things where GC might indeed be problematic and/or impractical.