[Matthias247]

I would bet it's just familiarity. Most people that are interested in PCs have heard of Linux, and know that it has a Kernel. Or they even have already compiled kernels themselves. Whereas LLVM is more of a thing that is known to a subset of software developers.

I'm personally happy to see the LLVM tests, since it gives a good impression on what benefits software developer which work on big native code-bases can expect.

[cesarb]

I think it's path dependence. The kernel has a large number of compile-time configuration options; back in the 90s, it was common to compile your own kernel, after tuning these options to your own particular hardware. It was also common to get new kernel releases (as a source code .tar.gz, or as a patch to the previous source code .tar.gz) directly from kernel.org, instead of waiting for the next release of the distribution you were using. So the time it took to compile a new version of the kernel for your machine was something many Linux users had experience with, and it was clear when a machine was faster (perhaps as a consequence of having previously compiled and installed a newer kernel!) because it took less time to compile the kernel. To turn that into a benchmark was just a matter of standardizing on a kernel release and a set of configuration options.