For some reason kubernetes community doesn't like docker. I'm not sure why, though. Afaik Docker was what started the whole container hype, and docker-swarm having some nice features like dependency tracking that I'm personally missing in kubernetes (there's probably a project for that already which I haven't tried out yet, tho).
My understanding is that Docker was the preferred container engine for k8s, but Docker did not reciprocate the interest, instead preferring to push Swarm.
Having supported Docker in production, I've found that the stability wasn't quite where it needed to be, but Docker Inc. seemed more interested in adding more and more features rather than stabilizing what they already had. I would definitely welcome a less complex base for running containers in production.
Docker was at times contentious, like when they injected an embedded DNS server into containers of some network types and refused to add a switch to turn that off:
That and, a few years ago, the amount of new bugs that would show up when they had to validate a new version of Docker for kubernetes to support. Eventually Docker adopted a less stressful, dual release cycle.
Google has internal equivalent of docker which they use instead, and they are one of biggest contributors to kubernetes. Im sure there are more reasons though.
