However, the remaining 2 systems are running IBM AIX which is a Unix variant.
In 2003, 96% of supercomputers used Unix, compared to 0.04% currently.
Why does this happen?

There are many reasons.
Modularity of Linux: This is one of the most important factors.
Thanks to this modular design, which can better optimize resources and reduce energy consumption.

So, all the things depends on the effort that is applied.
Add a new computer to the data pipe that allows scalable processing capability.
No need to restart: Having on a supercomputer waiting for the restart could take hours.
