The main issue was shipping it without proper runtime support, and even nowadays async/await is synonym with Tokio.

Look at .NET, it took almost a decade to sort out async/await across all platform and language layers, and even today there are a few gotchas.

https://github.com/gerardo-lijs/Asynchronous-Programming

Rust still has a similar path to trail, with async traits, better Pin ergonomics, async lambdas, async loops,..... (yes I know some of them have been dealt with).