In every ZFS on Linux discussion, there's a question like this. The short answer is that nobody really wants to be forced to run FreeBSD just for ZFS...
Also consider hardware support, mindshare and the flexibility/need to use more of the OS than just the filesystem.
I began with ZFS on Solaris, then Nexenta... then jumped to ZFS on Linux in 2012. FreeBSD was a non-starter because my hardware platform (HPE ProLiant) was poorly supported under FreeBSD. That killed it as an option.
Also consider hardware support, mindshare and the flexibility/need to use more of the OS than just the filesystem.
I began with ZFS on Solaris, then Nexenta... then jumped to ZFS on Linux in 2012. FreeBSD was a non-starter because my hardware platform (HPE ProLiant) was poorly supported under FreeBSD. That killed it as an option.