My guess is that, in light of the insanely limited memories of the machines they were working with, they had to leave out any feature that wasn't absolutely nec

My guess is that, in light of the insanely limited memories of the machines they were working with, they had to leave out any feature that wasn't absolutely necessary. Maybe the benefits of coroutines weren't as obvious then, or maybe they didn't seem to mesh well with the rest of UNIX ("why not just fork") Then, when UNIX and C exploded into far bigger things than they had probably ever dreamed of, it was too late. I think C would be a very different language if you had explicitly given Dennis Ritchie the task of writing the eternal lingua franca of programming with at least 80's-level computing resources at his disposal, but that's history for you.