[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.