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.
> Because ZFS is complicated to use
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?