The main issue is that there is no consensus yet on how the API should look like. Considering Rust's backward and forward compatibility promises, committing to an API is an extremely serious step, all the more difficult when it's not clear what the API should be.
Instead, Rust is waiting for patterns to emerge in the ecosystem, so that the universal API is compatible with all of them. It's a much safer route, but it also means waiting for the ecosystem to fragment. This is where we are now, but it's necessary and will get better over time.
Edit: async readers/writers are a good example, because it seemed trivial to set in stone, except now we have io_uring that might require the API to move to the kernel instead of just holding &mut.
Instead, Rust is waiting for patterns to emerge in the ecosystem, so that the universal API is compatible with all of them. It's a much safer route, but it also means waiting for the ecosystem to fragment. This is where we are now, but it's necessary and will get better over time.
Edit: async readers/writers are a good example, because it seemed trivial to set in stone, except now we have io_uring that might require the API to move to the kernel instead of just holding &mut.