Look at the history: In the beginning Kubernetes didn't have deployments - only replicasets, it didn't have ingress, it lacked the cloud provider integration you have today...
It was barebones like Mesos but they managed to focus on the right things early on. Once these features and a bit of an ecosystem started to build, the question if we should stay on Mesos with a bunch of homegrown management tooling or move to Kubernetes wasn't even a decision anymore. There was still a significant amount of code which we later just deleted because it became a first class feature or the community implemented something much better.
It was barebones like Mesos but they managed to focus on the right things early on. Once these features and a bit of an ecosystem started to build, the question if we should stay on Mesos with a bunch of homegrown management tooling or move to Kubernetes wasn't even a decision anymore. There was still a significant amount of code which we later just deleted because it became a first class feature or the community implemented something much better.