The licensing issues don’t help. Consider that most Linux machines are in commercial settings (Google runs more of them than all individuals combined) and they don’t need Oracle on their backs.
There is nothing complicated about having zfs be your filesystem. You can use its extra features like snapshots, send/receive, etc, but only if you wanted to. Otherwise it's just a filesystem, there is nothing to use as such, so nothing complicated.
> and isn’t necessary for most workloads.
Not sure what that means. What workload wouldn't benefit from a reliable filesystem?
Linux is the most common platform and many Linux distributions have an issue with license compatibility (or perceived one), which hinders adoption.
In the end, I think it's momentum and licensing. AFAICT btrfs only exists because ZFS can't be shipped in Linux kernel. Defaults are insanely powerful, and ext4/xfs is good enough for a lot of use cases. If ZFS were BSD or GPL licensed, I think every Linux distro would ship with it as an option, if not a default.