Because they have their own drawbacks. To make them really useful, you need a resizable stack. Something that's a no-go for a runtime-less language like Rust.

You may also need to setup a large stack frame for each C FFI call.

Rust originally came with a green thread library as part of its primary concurrency story but it was removed pre-1.0 because it imposed unacceptable constraints on code that didn’t use it (it’s very much not a zero cost abstraction).

As an Elixir + Erlang developer I agree it’s a great programming model for many applications, it just wasn’t right for the Rust stdlib.

One of Rust's central design goals is to allow zero cost abstractions. Unifying the async model by basically treating all code as being possibly async would make that very challenging, if not impossible. Could be an interesting idea, but not currently tenable.

One problem I have with systems like gevent is that it can make it much harder to look at some code and figure out what execution model it's going to run with. Early Rust actually did have a N:M threading model as part of its runtime, but it was dropped.

I think one thing Rust could do to make async feel less like an MVP is to ship a default executor, much like it has a default allocator.