Support for Docker was a correct market move for Cloud to adopt users that were already familiar with a tech base.

But divorcing their API from that tech base is also a move to support Cloud users---they don't want the story for big companies to be "If you want to use Kubernetes, you must also attach to Docker." That cuts potential customers out of the market who want to use Kubernetes but may have a reason they can't use Docker (even if that reason is simply strategic).

Google Cloud's business model walks a tightrope between usability and flexibility. Really, all the cloud vendors do, to varying degrees of success.