So per [1] it looks like Docker Hub launched in mid-2014. Admittedly my earliest pushes to there are more like the late-2015 timeframe, but definitely by that point I was pushing to namespace under my username. My impression had always been that the structure of Docker Hub was meant to mirror Github in terms of an org/repo scheme.

[1]: https://www.docker.com/blog/announcing-docker-hub-and-offici...