The first misunderstanding you seem to have here is that this is a site about GNU/Linux. It isn't, it is a site about any *nix operating system, including but certainly not limited to GNU/Linux. We have many questions here about the BSDs, AIX, Solaris, MacOS, and various others which have nothing to do with the GNU ecosystem. So renaming us to "GNU Stack Exchange" would be a huge reduction and limitation of the site's scope.
Even within the narrow scope of Linux, we absolutely do support questions about the kernel itself so again, we wouldn't want to limit the site to only GNU-related questions.
Finally, for the larger Linux vs GNU/Linux debate, I am afraid this has largely been resolved. The vast, vast majority of people who use GNU/Linux refer to it as Linux, and the term "Linux" has come to mean both "the Linux kernel" and "the operating system built using the GNU tools and the Linux kernel". While there are still some who object to this, most of the Linux world considers the issue closed. We all know the FSF's position on the name, and they aren't wrong, but the simple fact is that "GNU/Linux" is a mouthful and cumbersome and just isn't what actual people who use the system call it.
So, to summarize, the reasons we aren't called "GNU/Linux Stack Exchange" are:
- we are about much more than just the GNU tools.
- The Linux kernel itself is absolutely on topic.
- Non-Linux as well as non-GNU systems are absolutely on topic.
- Everyone, except awith very few exceptions, calls it "Linux", this is a non-issue.