Earlier quoted context omitted.
They have waaaaaaay more marketing than OpenStack. They've probably written and sponsored more PR fluff than code for Docker. And it's easier to deploy.
I agree that Docker is easier to deploy than Openstack :) However... It always astounds me how some people massively over-estimate the size and influence of Docker's marketing... Why yes, of course! The way we got Google, Microsoft, Amazon and IBM to integrate it in their products is by ghost-writing PR fluff. That's also how we got 600 people to contribute 9,000+ pull requests over 18 months [1] [2] [3]. Not bad for…
Docker resonates well with people because it focuses on aspects of virtualization that people care: development and deployment. It alleviates the need of using complicated configuration management tools by providing layered images, and encourages fine grained containers by supporting first class volume sharing.
The fact that it integrates well with other virtualization stacks is a proof that for a good part it's orthogonal to them.