The reason is that the Linux and FreeBSD kernels are vastly different. The FreeBSD kernel is more similar to Solaris than Linux and this is what ZFS was develop

The reason is that the Linux and FreeBSD kernels are vastly different. The FreeBSD kernel is more similar to Solaris than Linux and this is what ZFS was developed for. So in the end you have way more kernel "workarounds" for ZFS on Linux than the FreeBSD implementation. Especially the virtual memory implementation on Linux behaves very differently and has caused many issues over time, but I think it is very good now.