POWER is supported on linux to the point where it runs, but it doesn't come close to fully leveraging the hardware. AIX does. Stuff like hardware accelerators, transactional memory, hardware counters, reliability monitoring and self healing, etc. Lots of stuff left on the table because it doesn't perfectly overlap the x86, and it would be a massive undertaking to correct that in the kernel. You'd think that wouldn't be the way it is with IBM and RH... but I suspect there are some market segmentation ideas informing those decisions.