Suoer-computing is a pain-in-the-ass, so my guess is some combination of SUSE picking up top talent that left other Linux vendors as IBM has been purchasing them, and SUSE just being willing to put in the extra work for the added brand recognition.

Because all the Arch consultants were busy posting on the internet.

Because Cray have CrayOS which is a slightly modded version of SuSE. Why did Cray choose SUSE probably because the licensed support was cheaper that RHEL 😂