Kubernetes was originally written in Java and rewritten in Go after some Go advocates joined the team.
Docker was originally written in Python and rewritten in Go after some Go advocates joined the team.
Don't mix technical capabilities with people wanting to refresh their CVs.
Just to be clear, this didn't go very far. It's more like Brendan was willing to write Java, and a lot of other folks hate it. I don't think any of the core folks were Go advocates, just most of "us" were C/C++ people. Docker itself being in Go, meant it would lower a lot of friction, and besides we had great tooling for Go.
tl;dr: It was really "Should we (re)write this in C++? Eh, how about Go?".
And to come back to my point, if Docker or Kubernetes were invented today, I bet they would be using Rust or Zig instead of Go.