As someone who's used btrfs and zfs, btrfs today is still a pale imitation of what zfs was at launch. btrfs feels like people wanted to make a filesystem which had all the 'pretty neat' features of ZFS but without the tightly coupled management and benefits of ZFS (like volume management + RAID + encryption + compression all being managed through one tool).

btrfs ended up being basically a consumer version of a few of ZFS's most visible features - transparent compression, subvolumes, and copy-on-write - which then farms out the other most useful features - RAID + volume management + encryption - to the existing tooling. Makes sense for a smaller-scale project, no shade, but it definitely misses the mark if you've spent time on ZFS systems taking advantage of what ZFS has to offer (like recursive snapshots or vdevs).

I don't know the whole history of btrfs (although its primary maintainer was I think at Oracle at one point) but ZFS was definitely built from a hierarchical storage enterprise mindset.

Given different historical circumstances, it could probably have picked up more traction. But not sure what would change today even with an Oracle licensing change.