I think the main issue here is Btrfs is still developing. Its kernel doc file still says it's for benchmarking and review. [1] CoreOS devs decided to switch from Btrfs to overlay(fs) about 17 months ago. That's a long, long time in Btrfs development "years" that's how much development happens on Btrfs. But I can't say if CoreOS would, had today's Btrfs been what they were using in 2014, would have changed their decision.
RH/Fedora are very dm/LVM thinp snapshots with XFS centric for backing their containers. I think what you're seeing is distros are doing something different with their container backing approach in order to differentiate from other distros. Maybe it's a stab in the dark or spaghetti on the wall approach but in the end all of these storage backends are going to mature a lot in the interim, so ultimately it'll be good for everyone.
[1] https://git.kernel.org/cgit/linux/kernel/git/stable/linux-st...
Btrfs has been in beta for how many years now?
ZFS is protecting data in enterprise production environments since 2006 (Solaris 10 update 2).
Btrfs is not beta, see the discussion re: maturity below.
ZFS is an excellent filesystem, btrfs is an excellent filesystem. There is room for both excellent options for users.
For development containers, systemd-nspawn has had support for btrfs snapshots since 2014 or 2015. Simple to get going if you're on a Linux box with systemd, no other daemons or tools required.
Not sure if your container tool supports btrfs snapshots of course, but it's conceptually simple, right?