It seems quite clear that rust users use hyper but few of them want to work on making it work for a C project like curl, and among existing curl users there is virtually no interest in hyper. The overlap in the Venn diagram of the two universes is not big enough.
The most important part of curl is the library, not the CLI tool. And the TLS backend is very much relevant due to varying features supported, different licenses involved, and varying levels of ease when it comes to building and distribution.
That’s when you’re providing binaries. Otherwise, you would have to be wary of potential compatibility issues with
libcurlpackages provided by different Linux distributions for example.See CURLOPT_ECH for a recent and still evolving example of this.