I think LordChambers has it right. The AI will pay for peace up to a point. Beyond that point, though, it decides that you're going to kill it anyway, and won't give you anything.
Which -- let's be honest -- is often exactly right. If the AI would give me those three techs, I'd probably take them, wait ten turns, and then finish him off.
If you think about it, this behavior IS self-preservation. If I knock Monty down to three cities, and he offers me a tech and some gold for peace, and I know I'll get nothing if I keep attacking him... well, maybe I'll settle for peace.
Anyway, key point: if you knock the AI down too far, it'll stop paying for peace.
Waldo