Just my cynical view, but I'm thinking it's because is got a friendly looking whale as a logo and the developers took the approach of making it friendly to use for someone who doesn't understand the underlying quite complex technology.

I was involved in making the decision about introducing containers to my workplace, my original recommendation was to just go with chroots or buy into something that we could get vendor support for on RHEL6 (so not docker) and when asked to explain how this would benefit out desired outcome, the docker website basically had copy-pasteable words that did that job for me, chroots however just described he technical implementation of a chroot.

So eventually I just went "fuck it, we'll run unsupported and take the risk, then well move N thousands of servers to RHEL7 because I don't want to have to explain to managers how cgroups and Linux kernel constructs work". About a year later most of the enterprise has a plan to move all their existing EL5/EL6 workloads to EL7 just in case they need to use docker some time in the future.

Which, if I'm honest, I kind of regret, but it doesn't impact me that much so meh.