What are the reasons that Kubernetes no longer uses Docker as its container engine?

Kubernetes stopped using Docker as a container engine (runtime) for several reasons:

1. Standardization through CRI (Container Runtime Interface): Kubernetes implemented CRI — a standard interface for interacting with container runtimes. This allowed the use of different engines without depending on a specific Docker implementation.

2. Complexity of Docker Engine: Docker is not only a container runtime but also a set of tools (daemon, CLI, image building, etc.). Kubernetes only needs part of this functionality — running containers.

3. Transition to lighter and more specialized runtimes: Such as containerd and CRI-O, which are simpler, lighter, and better integrated with Kubernetes via CRI.

4. Support and development: Docker Engine is not optimized for large-scale Kubernetes operation, whereas containerd and CRI-O are developed with a focus on Kubernetes.

As a result, from Kubernetes version 1.20 and above, Docker as a runtime was declared deprecated, and from 1.24 it was completely removed, replaced by containerd or CRI-O, which improved performance and simplified architecture.