Coroutines and first-class continuations make the implementation of non-local transfer of control much more complicated. Their inclusion would have made staple

Coroutines and first-class continuations make the implementation of non-local transfer of control much more complicated. Their inclusion would have made staples like unwind-protect, conditions, and underlying them try/catch, much more difficult to implement, if not nearly impossible to get right. I personally think that those language features are worth it and that this particular trade-off was well made.