Everyone has mentioned the license, which is the reason ZFS isn't everywhere. I just wanted to add: Don't let the license scare you away. The CDDL is actually a nice compromise between copyleft (GPL) and copyfree (BSD). The CDDL is essentially the same as GPL except the GPL applies to everything it touches (include a single GPL *.c file in your project and now your entire project is subject to the GPL). Whereas the CDDL only applies to the CDDL-licensed files themselves. So you can include ZFS in your project without making your entire project subject to the CDDL.

The issue is, the GPL wants to spread to everything it touches (including the CDDL-licensed ZFS files), which would make the ZFS files subject to both the GPL and the CDDL. But the CDDL and the GPL are incompatible, which is why Linux can't include ZFS whereas more permissively licensed projects like the BSDs can.

So it's an unfortunate situation, but ZFS' license is not "bad". It's just the GPL and the CDDL are incompatible.

Because ZFS is complicated to use and isn’t necessary for most workloads.

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.

I would say (in no particular order) that the CDDL license is not innocent, ext4 is good enough for many usages, cost ? ZFS is RAM hungry ... but ZFS is well present where it means most (NASes , Proxmox , backups to tell some)

Primarily licensing, IMO. Its license is incompatible with inclusion in the Linux kernel. Ubuntu ships with native support for it anyway; I guess they're not worried about legal issues. It's completely functional in Linux, you just have to take an extra step to install it, which is enough to keep it from mass non-techie adoption.

Totally worth it for NAS. I think there are more systems that utilize its advanced dataset features in the BSD world.