I know several people on various C++ committees and by and large their opinion is, we evolve the language and library to give existing projects incremental improvements without asking them to rewrite them, but if you are starting with a new project C++ is often a subpar option. From that perspective I get why they'd be hesitant about efforts like Circle. Circle and co. ask developers to rewrite their code, in something that looks very different to normal C++ - whatever normal C++ even is, given the multitude of dialects out there - can't seamlessly interop with existing code, needs a new incompatible standard library, that as of now doesn't even exist. At which point, honestly just rewrite it in Rust instead of going through the painful exercise to use something that's 10+ years behind where Rust is today in terms of DX, tooling and ecosystem.
But all that doesn't explain why at the very top, even mentioning Rust as an alternative seems taboo, idk.