This was one of Sun's last middle fingers to the FOSS community. They knew by licensing ZFS under the CDDL there was no way Linux users, specifically RHEL customers could benefit from it. Hopefully Apple won't follow suit with APFS.
GPL and AGPL differ vastly in a few important aspects that make GPL pretty OK for commercial companies, and AGPL an absolute no-no.
Some prominent former Sun folks have publicly claimed the GPL incompatibility was intentional, other equally prominent folks have disagreed.