Docker got in as a container runtime, without any orchestration capabilities, then K8s came along to do orchestration but didn't provide it's own container runtime, so they used Docker.
After that there was some tension as Docker wanted to move "up the stack" to provide features like orchestration (with Docker Swarm) but the k8s community saw that as unecessary, they wanted a simple container runtime to sit under their orchestration layer.
Now we have options like cri-o or cri-containerd which are likely in the medium term to take over from Docker as the container runtime underneath Kubernetes installs. I'd expect that Docker will see more use in dev/test environments where full scale clusters are not required.
