It might be worthy of mention that when it comes to operating systems, the last two Unix systems have dropped out of the Top500 and so it is now 100% Linux.
I'm sure this was because at one point CRAY built one for the US DoD/DoE leveraging AMD Opertons & SUSE, and of course CRAY used to be the big name in supercomputers so I'm sure that influenced other builders choices. Then I recall the US DoD/DoE mandating a Linux OS engineered closer to home, so Red Hat got the nod.
So I wonder what the prevalence of the various distros in supercomputing is now?