ZFS never came from FreeBSD -- its origin is Solaris, and specifically from the tediously open-sourced and then thrown under a bus by Oracle OpenSolaris, forked into the Illumos project.
So FreeBSD's version has never been the "master copy", so to speak. In some ways the most sophisticated version has always been the (now very aggressively close sourced) Solaris 11 ZFS, which is very incompatible with the open source forks because of new features added.
FreeBSD switched from upstream Illumos to the OpenZFS/Linux sources because Illumos development has slowed to a crawl, while ZFS on Linux is ironically very active and has more and more corporate backing, while essentially all corporate backers of Illumos except maybe Joyent (?) are dead, as mentioned on the FreeBSD mailing list. This means in practice development on that fork of ZFS ceased as bugs piled up, since relatively few hobbyists are going to deep dive into something as complex as ZFS. [0]
FreeBSD gained several new features from Linux's ZFS that it simply didn't have before, like native encryption support in ZFS itself without needing Geli/GEOM. I've heard several FreeBSD users get excited about this, so I presume the wrapper wasn't well-loved. Other features mentioned in the mailing list are "multi modifier protection, project quotas, encrypted datasets, allocation classes, vectorized raidz, vectorized checksums, and various command line improvements". It's simply superior to the Illumos upstream, and will only improve over time while Illumos sadly stagnates into oblivion.
The kernel guys are never going to like it, and I fall closer to their camp. I'd rather use Linux's native layering mechanisms instead of using an omnibus package. But I also think ZFS on Linux is in practice very widely used and very mature. Same as the nvidia drivers, too -- people endlessly complain about them, but even Fedora adds a button in the GUI for installing 3rd party repos with the drivers, and my nvidia blobs have survived for 4 OS updates without breaking. There are several OSes like Pop_OS! that just include them OOTB.
[0] https://lists.freebsd.org/pipermail/freebsd-current/2018-Dec...