Docker is just an abstraction & automation layer on top of system-provided containers.
The technical limitation is that OS X does not support operating system-level virtualization, like containers in Linux or jails in FreeBSD (even despite the fact that OS X is a half-blood BSD).
But even despite that, Docker aims to provide an easy way to share container images, and to make sure that any Docker image would work on any Docker-compatible system, they had to limit it only to Linux. (That is probably why Docker doesn't support FreeBSD and friends, which do have containers.)
UPDATE: you can use projects like docker-osx or boot2docker to enjoy Docker on your OS X machine almost seamlessly (both create a Linux virtual machine behind the scenes.)
