I only mentioned Linux because I knew that my argument wouldn't work for BSDs! But in my defense the situation, at least originally, was more political than technical (at least as far as FreeBSD was concerned, I don't follow the other BSDs too closely). GCC switching to the GPLv3 was a big no-no for many people in the BSD world. As a result they were at the forefront to port the system to clang since they were stuck with an obsolete GPLv2 GCC version.