Ketamine is a dissociative that's used as an anesthetic. General anesthesia is a different beast. The precise mechanism GAs use isn't understood, but they cause slow waves of electrical activity to wash over the brain, constantly resetting neural networks and preventing communication, leading to coma. This includes breathing, which is why the patient needs to be intubated.

Ketamine doesn't stop respiration. It causes profound dissociation and hallucinations, but I don't think it causes coma. It's definitely doing different things than GAs, at least.

So it doesn't seem unreasonable that ketamine wouldn't work if you're in a coma. I'd be surprised if LSD did either. Or anything, for that matter. There's not a lot metabolically happening in a neuron under coma.

Propofol works even on Venus flytrap plants. That's how deep this particular 'off' switch seems to be wired in. (It's blocking the presynaptic sodium and calcium channels.)

I think in other words, stuff like ketamine and LSD don't perform magic all on their own. You have to be consciously experiencing it in order for them to have any effect on you.