History. RMS was worried that "someone" (basically, Intel) would be able to take the gcc frontend and attach the icc (very fast, proprietary) backend to it, thus creating a compiler which could compile the whole GNU stack, producing very fast code. This would entice people to use a non-free compiler.
Since then LLVM came along and made the whole point moot, since it has a modular front end which can parse all the gcc extensions.
