It was never emphasized, and probably rarely used by their customers. Prior RHEL release used ext4, then xfs by default. People who run RHEL don't want to think about what filesystem to use. They want standard, stable, reliable -- not dozens of options and features.

When you're pushing the container stuff, Btrfs becomes extraordinarily valuable. Being able to snapshot a base thing and then layer on top, and do trivial rebasing, as well as cheap replication of container volumes is _very_ useful.

On my Fedora and EL7 systems, I use Btrfs for the OS and for my containers for those purposes. It's awesome.

Slight correction: we use btrfs for the disk image backing the user's crostini state only. The rest of Chrome OS, and the crostini VM rootfs, still use ext4.