K8S maintains the internal state of the system in JSON which is easy to convert to YAML which is seen as more readable in comparison. I think their choice to support JSON and YAML as an interface for configuring K8S is because of this.

I'd also guess that the expectation was that abstractions would form around YAML to make it more powerful. Helm uses Go templating language which supports logic and loops (but is very unpleasant to write complex configs with) and the operator pattern is also popular for more advanced cases. Ultimately both end up exposing some JSON/YAML interface for configuring, though. It is up to you to decide how you create that JSON/YAML.