Go is huge in the Kubernetes world... mainly because both are Google inventions. But almost all of the third-party integration I've seen in k8s has involved Go.
Kubernetes actually started out in Java, and was only rewritten into Go later (before 1.0 though). The choice of Go could also have been influenced by Docker being written in Go (which also meant that some Docker-related libraries were already available for Kubernetes to use).
The super early Java prototype of Kubernetes lived for about a week or two and never saw a public repository. It's kind of misleading to say it "started out in Java".