Because the CDDL (by design, as inherited from the MPL 1.x) is not compatible with the GPL, due to the fact that both try to enforce their own copyleft without

Because the CDDL (by design, as inherited from the MPL 1.x) is not compatible with the GPL, due to the fact that both try to enforce their own copyleft without allowing relicensing (i.e the MPL2 avoids this thanks to a annex clause that allows you to consider everything as covered by the GPL instead). You can't cover the ZFS modules with the GPL in the final agglomerated result (i.e. kernel + module), so it's not allowed technically by the kernel license (see the difference between the LGPL and the GPL for more info). In practice no one ever questioned this in court, so who knows what the final result would be. FreeBSD uses a super weak permissive BSD license instead, so there is no conflict because it leaves whatever code you're trying to add to it alone, without trying to enforce any additional terms on it.