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.