SPARC is a neat ISA, but the concept at the core: register windows, did not take off.

OpenPOWER: It may have to do with the size of the ISA when it started to be open.

RISC-V, when announced was a small ISA. That means making chips is easier.

It could just be the new hawtness effect. Look at how many people use NoSQL when SQL would work just fine ;).