We already have the actor model in Rust, via threads. Some people want better performance than what threads can provide. M:N was tried in Rust and the performance was found to be insufficient (and there were other downsides too). So async/await was the next best option.

If you want the actor model, you can use it! There is nothing stopping you from going thread-per-connection in Rust. It's quite fast on Linux for all but the most demanding workloads. If I were writing an ordinary network service expecting moderate traffic, I'd probably go thread-per-connection.