It uses the GNU GPL version 3, which is incompatible with the use cases of many of BSD's "customers". Notably, Apple has refused to use the GPLv3 versions of GC

It uses the GNU GPL version 3, which is incompatible with the use cases of many of BSD's "customers". Notably, Apple has refused to use the GPLv3 versions of GCC in Mac OS X, and FreeBSD has followed suit by staying with GCC 4.2 until clang is considered finished. It also tends to have problems on less popular architectures that Net/OpenBSD still consider important.