I have often wondered this as well and never quite found a fully satisfactory answer. I imagine that it has to do with ease of implementation, although there's

I have often wondered this as well and never quite found a fully satisfactory answer. I imagine that it has to do with ease of implementation, although there's an obvious and efficient implementation of making coroutines one-to-one with stacks and allowing them to yield between each other. In constrained environments like JVM and CLR, maybe that can't be done, which might explain C#'s choice of async/await. Perhaps in Rust it has to do with the interaction with the borrow checker?