Rust actually used to use conditions for its I/O errors, but they were removed as part of the “new runtime” project in 2013, for reasons that are dimmed in my memory by time, but I think they included: quite a lot of complexity, including in ways that had negative performance implications; lack of clarity about where errors could occur; lack of ability to distinguish between errors by source in the handler; more limited potential than hoped in ways that were probably connected to Rust’s ownership orientation; unfamiliarity to users (and Rust had already spent its weirdness budget); and that the actual power of conditions was almost entirely unused (I honestly don’t think I saw any production-like code use handlers for anything interesting, ever).

You can still readily express the concept of conditions in Rust, but it’s no longer baked into the standard library.