And GCC targets fifty billion different instruction sets.

doesn't clang also support many backends?

Many, but not as many as gcc. Gcc being the only game in town for about thirty years gave it a massive head start, and gcc existed during an explosion in different isas and architectures. Every obscure processor in the world seems to have a gcc backend.

Clang much less so. But clang supports all the modern “important” isas, so as a practical matter, it doesn’t matter that much for everyday use.

My impression is, GCC development team is more open to mainlining support for obscure architectures, Clang/LLVM require stronger evidence of demand and maintenance commitment.

Of course, 99.9% of people don't use any obscure architecture, so that's irrelevant to them.

due to clang's license, some vendors like to release their own proprietary version of clang for an architecture (though in cases I'm aware of, parallel unofficial open source support exists as well, though I don't know if this is always true).