Super-computers have become massively parallel, with hundreds or even thousands of CPUs. Legacy software providers typically set license terms according to the number of CPUs, and software license fees could get prohibitively expensive.
Yes, Linux can be easily customized. But, if you are building a massively parallel with nodes of 2-16 CPUs, you really don't need much customization: the kernel already has everything you need. You might need to develop or tweak a device driver for your interconnect, but that's easy to do, and possible on legacy OS's as well.
Bump FYI !