More seriously though, that's a fair question. Anything that isn't the web has a lot of catching up to do before it can compare to the web.
(Though browsers are increasingly relying on rust frameworks for low-level work, so there's some potential for code reuse.)
My gut feeling answer would be "because things built for the web have a tendency to fail for no reason, and things built in Rust have a tendency to work on the first try". I'd have a hard time backing up that gut feeling with hard numbers, but I think it's a shared one. Web techs tend to have all sort of legacy cruft and small papercuts and stuff. Mature Rust crates are easy to install, work immediately and have an intuitive API. My hope is to get to a point where developing Rust GUI is just easy, no papercuts.
(But again, lots of catching up to do first.)