It may be because of implementation complexity? C was designed with the compiler writer in mind (which is why so many things are undefined... to make it easier for compiler writers), so if co-routines are hard to implement then that might be an explanation. Anyways that's mostly pure speculation on my part.
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...but thats the thing, on the PDP the crux of it was essentially one instruction. May be the worry was that porting this to other architectures of the time wasnt feasible or easy.
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.