Solaris was more stable in the mid-90s but that advantage had flipped by the turn of the century. It was an order of magnitude more expensive to support, especially at scale because Linux was far ahead on packaging and configuration management by 1998 or so, which in practice meant that no two Solaris boxes were the same unless your organization was willing to spend a lot of money on staffing.

Solaris 10 was intended as a catch-up release but the package manager took years to mature. I remember getting racks of systems in the mid-2000s where simply running the updater on brand new servers rendered them unbootable. By then the writing was on the wall.

SPARC was similar: the hardware had some neat aspects but what they really needed was the equivalent of a first-class LLVM backend now. Statistically nobody was hand-tuning assembly code to try to get it to be competitive with x86, especially since that meant not only your own code but most other libraries. The reason ARM is doing so well now is that two decades where the most popular computing devices are phones means that you can just assume that anything popular will compile and run well on ARM.