During the design phase of Rust async there were no async embedded code written to be inspired. For systems with tight memory budget it is common to pre-allocate everything often using custom bump allocation or split memory into few regions for fixed-sized things and allocate from those.

And then the need to poll features by the runtime means that async in Rust requires non-trivial runtime going against the desire to avoid abstractions in the embedded.

Async without polling while stack-unfriendly requires less runtime. And if Rust supported type-safe region-based allocators when a bunch of things are allocated one by one and then released at once it could be a better fit for the embedded world.