OpenZFS is a fork of the Sun/Oracle release of ZFS. The OpenZFS maintainers cannot change the license without consent of the copyright owners, and Oracle is unlikely to consent. Oracle could potentially change the terms in a future release of CDDL, and that might work, too.

Similarly, Linux probably has too many contributors, some of which are no longer living, to come to an agreement on a fixed license either.

Although, license changes do happen; OpenSSL did one recently, but I think there are fewer contributors.