I think this has to be put against the larger context of Rust… fans that push very hard for the entire world to be rewritten in Rust. Just look at how many Rust projects where their front page / README has the first line as "written in Rust" as if that's the number 1 feature a user should care about. As such I think there was an outsized voice of re-writting cURL in Rust and whatnot as occasionally there will be buffer overflow bugs that admittedly would not have existed if the code in question was written in Rust. That's why there was the experiment to add a new backend using Hyper because in theory they should not be prone to the same class of memory bugs.
As for why not making it the default, the post already made it clear? The new backend did not pass all the tests and therefore could not possibly be the default backend until that was fixed and turned out fixing the last few issues were quite hard. cURL has also supported different backends for a while now. For HTTP/3 for example you can swap out the backend for something else. This is like driving a car. If you literally don't care at all, just use the stock tires/engines and it will drive fine. But sometimes you may care and swap out the stock components for something else that suits your needs, and cURL allows that. cURL / libcurl gives a stable ABI as a glue but it's intentional that you can swap out the backend.
As for why not making it the default, the post already made it clear? The new backend did not pass all the tests and therefore could not possibly be the default backend until that was fixed and turned out fixing the last few issues were quite hard. cURL has also supported different backends for a while now. For HTTP/3 for example you can swap out the backend for something else. This is like driving a car. If you literally don't care at all, just use the stock tires/engines and it will drive fine. But sometimes you may care and swap out the stock components for something else that suits your needs, and cURL allows that. cURL / libcurl gives a stable ABI as a glue but it's intentional that you can swap out the backend.