The full power of F# async workflows can be had with the same syntax async/await has now. Basically async workflows could have been a more convenient syntax for let! in F# async workflows, in the sense that you can directly apply await inside an expression instead of having to insert a let statement:

    ... Foo(await m) ...

Instead of, in F#:

    let! x = m
    ... foo x ...

It is certainly possible that the C# designers did recognize the possibility to make await work for any monad, but decided against it because it could possibly result in confusing code or worse performance.

If that's the case then it would be great to hear the reason from one of you who are on the C# design team and reading news.yc :)