AUFS isn't included in the default setup of many Linux distributions. CoreOS, CentOS, RHEL do not have it. If I recall correctly, CentOS and RHEL will use the device-mapper driver by default and CoreOS the btrfs. I would speculate that the docker team gave so much attention to AUFS to lower the barrier to entry but it isn't the only option.

My choice is btrfs and it has been rock solid for me since the start. Of course one has to know a bit about btrfs and always leave a healthy percentage (I'd say 10% or more) of the filesystem free.

I think AUFS and overlay2 do have an advantage of their own though (apart the works-out-of-the-box). Due to the way they work, you can have shared memory between containers. Thus if I had to run many containers with a JVM app, I would give them a try to lower the tax on RAM.