The fact that ZFS isn’t massively popular after close to 30 years, and after nearly 20 years of availability via OSS, suggests that the benefits aren’t really that meaningful to most users.
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.