The version of ZFS that everybody (besides the dwindling number of Oracle Solaris customers) uses now, OpenZFS, has been maintained completely independently of Oracle since they shut down OpenSolaris in 2010. This means that Oracle relicensing ZFS wouldn't do anything to help with getting it integrated into the Linux kernel, since there's been hundreds of independent contributors to ZFS since then who all own their own copyrights. Because ZFS is licensed under the CDDL, which has an automatic upgrade clause, Oracle could simply copy/paste the GPLv2 license text and call it "CDDL v2" if they wanted to make ZFS able to be included in Linux.
Swapping to an entirely new license rather than adding one sentence to the existing one is not simpler either in terms of linguistics or getting approval from their army of lawyers.
It would require getting all past contributors _and_ the current copyright owner to agree. Normally I'd agree that getting all past contributors to agree is the hard part, but the copyright owner here is Oracle. I'd have more confidence in getting the rest of the contributors to agree to a license in Pig Latin than getting Oracle to make literally any change to the license for this.
How about a simpler solution, just relicense everything to BSD / MIT.