For JS, cynical take would be that Javascript is just being consistent in making every wrong choice in language design. More serious (and more correct) answer is that, being single-threaded, adding any kind of threads would be pretty adventurous. Async/await as a sugar for promises is a decent improvement over using promises directly, so it's not so bad of a choice after all.
As for C#, some kind of fibers appear to have been considered but deemed too hard or not worth the trade-offs. See here: https://github.com/dotnet/runtime/issues/11084
The acceptable alternative was, apparently, forcing to programmers to litter their code with words such as async/await/task in-between the words and symbols that actually describe the purpose of their program, should they want to take advantage of non-blocking IO. All the way throughout the call-stack,
Read the rest
nonetheless. And you also get crap like this: https://github.com/dotnet/corefx/pull/4868