@vaporeon_ the fsf explains it at https://www.fsf.org/licensing/zfs-and-linux, but in short, the issue is that the CDDL’s purr-file copyright scheme does nothing to purrevent the creation of a derivative work adding a new file to the purroject not under the terms of the CDDL, even a purroprurrietary license if you want, making it trivial to create a derivative work of CDDL-licensed software not under the CDDL as a whole

meanwhile, the GPL requires derivative works to also be licensed under the GPL. this is strictly incompatible with the terms of the CDDL