This is a question I've been asking myself lately. I went down a bit of a research rabbit hole and came out with some actual appreciation for the architecture. It's basically Berkley RISC - the ORIGINAL RISC - but commercialized by Sun. But they did more than that, they actually made the ISA completely free and open source. No royalties required. And they even released most of two of their at the time high-end 64-bit CPUs under a GPL license as well, these being the UltraSPARC T1 and UltraSPARC T2. Oracle still has all the files available for download as well!
And then in 2017, something happened. Both Oracle and Fujitsu, the main companies driving SPARC, just... stopped. They released one last iteration in September, then Oracle just laid off the entire SPARC hardware design division. Oracle wanted to shift to, uh, x86? Specifically Intel's x86? The ones that are cooking themselves?
Fujitsu, meanwhile, kept going with some incremental tweaks for a few more years, but then they went "fuck all UNIX" and partnered with ARM to make ARM CPUs instead. Which is the complete and total opposite of SPARC's model. And by that I mean licensing model, as ARM is basically an old clone of Berkley RISC that is totally proprietary and made by companies that don't like it when people want to use their devices.
I've looked up the list of companies that worked with and/or made SPARC CPUs. 99% of them are defunct now, namely being gobbled up by other companies who then switched to a different architecture, usually ARM or RISC-V, and setting fire to anything legacy. All that's actually left is an aerospace firm that makes radiation-hardened CPUs for space exploration, and a group in Russia that looks like to have also switched to something else, this time an in-house ISA.
The official site for the group that oversees the ISA, SPARC International, Inc., is kind of in a sad state. The security certificates expired in May of 2024, and there has been no new activity or postings since 2020. I don't know if they're still in operation. I'm honestly considering writing them a letter and asking if everything is okay there.
So seriously, what the hell happened? It's a CPU architecture that's very mature and is just as open, if not more open, as the younger RISC-V. It has everything you would need to make free software people salivate, while also handling larger workloads than Intel's CPUs and sip power while doing so. I mean sure, the main customer was people needing high end servers, so these server CPUs would be totally overkill, like telling people to be using Threadrippers or Xeons in their desktop machines for watching YouTube and playing Fortnite. But a cut-down version should make total sense! Hell, if you were to take the UltraSPARC T2, scale it down to one of TSMC's more recent production nodes to get more performance out of the design, and only provide 4 cores, that's more threads than an Intel Core i9 14900. And people are just, abandoning this?!
What the hell happened?
EDIT: Okay I'm editing this into the post because this is something I forgot to mention that really adds to the mystery. In terms of other open source implementations, there was also the S1 Core. It only had a single core from the UltraSPARC T1, but was much more simpler and had tooling that could be used from regular Linux instead of Solaris. But it's been vaporized from the internet. The official site now redirects to a completely unrelated RISC-V project, and nearly every mirror of the original project has bitrotted away. All that remains is an archive.org snapshot...