Today I read the article “What Would a Kubernetes 2.0 Look Like? Thoughts on what the next major version might be. And found this :)
"YAML is just too much for what we're trying to do with k8s and it's not a safe enough format. Indentation is error-prone, the files don't scale great (you really don't want a super long YAML file), debugging can be annoying. YAML has so many subtle behaviors outlined in its spec."
"HCL is already the format for Terraform, so at least we'd only have to hate one configuration language instead of two. It's strongly typed with explicit types. There's already good validation mechanisms. It is specifically designed to do the job that we are asking YAML to do and it's not much harder to read."
and realized that Kubernetes developers had the same thoughts about using YAML- but instead of HCL, they just invented their own HCL-like language: KYAML.