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.
At first I was disappointed to see this, but after looking into it it looks like they weren’t using hyper as a means to migrate the project to Rust. If they’re not going to move away from C, it seems like a fair decision.
I occasionally use curl myself but never really thought about what language(s) it used in its implementation. Curious if there are power curl users to whom its internal implementation details would really matter. For me its mainly just testing my own websites and vulns there are unlikely. If I was using curl to scrape a lot of sketchy sites I might care about its memory security more.
I generally assume utilities like curl are written in C.
curl in particular I believe stands for cURL.