GNU is the operating system, Linux is the kernel, both together get you a working system on which you can install all sorts of other programs.
nobody's crediting GNU for Linux.
the opposite happens when one refers to the GNU operating system running on top of the kernel Linux as if the whole thing was Linux.
we ask GNU to be credited for GNU, not for Linux.
the main reason for that is that, in order for GNU to fulfill its purpose of defending users' freedoms, it's important for users to realize that this is the system they use, so that they can learn why it was made, and why their freedom is important.
Linux doesn't share those goals, mission and values; it doesn't defend users' freedoms, it often stands against them, and it's proud of its unprincipled stance. there's nothing wrong with being proud of one's stance, but it would be self-defeating for people who care about users' freedoms to promote Linux's values and sideline GNU.