The CDDL is a file-scope weak copyleft license, and is (broadly) less restrictive than the GPL. As bunderbunder pointed out FreeBSD is not able to include GPL

The CDDL is a file-scope weak copyleft license, and is (broadly) less restrictive than the GPL. As bunderbunder pointed out FreeBSD is not able to include GPL code, because the GPL is not compatible with the distribution terms for the kernel.