Good question, but I'm not yet experienced enough in language design to give a coherent answer. And I'm not very familiar with delimited continuations. But I think it boils down to keeping the VM simple. Now it's just all closures, and I can focus on optimizing closures. Adding first class continuations would reauire me to also optimize those. Also having continuations makes optimizing closures harder, since it prevents certain assumptions. As long as closures and CPS enable everything I need, I'm not tempted to add another, more powrful structure.