Not a Rust designer, but a big motivation for Rust's async design was wanting it to work on embedded, meaning no malloc and no threads. This unfortunately precludes the vast majority of the design space here, from active futures as seen in JS/C#/Go to the actor model.
You can write code using the actor model with Tokio. But it's not natural to do so.
You can write code using the actor model with Tokio. But it's not natural to do so.