Re: Why no ARM servers?
ARM 64-bit support only emerged in 2011, and it's vastly different from the 32-bit ISA (I understand it's much more like MIPS).
This also came late to x86 with the Opteron in 2003.
MIPS owned supercomputing in the 90s starting with the 1991 release of the 64-bit R4000.
The ARM 32-bit ISA had design decisions that limited performance. I would say that Sophie's ISA was perfect for an '80s Acorn, but not so much for a Cray.
https://www.jwhitham.org//2016/02/risc-instruction-sets-i-have-known-and.html