Because even using only the term "Linux", one of the *major* selling points of Linux is that it is free/Free and open source. And one of the first things people learn about it is licensed under the "GPL". That should be enough. RMS's comments make me want to go out and clone my own set of UNIX utilities and write my own license called the "FPL" (Free Public License) but would follow the same rules of the GPL regarding keeping the source open and modifiable.
Since Linus is the copyright holder of the Linux kernel he could change it over to the FPL license and use FPL utilities to make up much of the OS. It is something he might seriously want to do with all of the unjustified bad mouthing he's received. Then where would GNU be?
Now, I would rather that RMS tone down his criticism of Linus and the naming of the Linux OS. And it wasn't Linus' who encouraged anyone to refer to the entire OS as "Linux". After all, he just wrote the kernel which he named Linux, it was all of the distro people that decided to call the OS "Linux", probably because before the distros the first thing you did was download the Linux kernel, then get the GNU pieces that were adapted to the Linux kernel one by one (which wasn't the entire system in the beginning). It was "natural" to refer to the system as "Linux", not imposed.
Changing it now is just grand standing in my opinion. Linux already significantly promotes GNU and Free Software, no need for a name change.