Jeff Barr on Thinking on Your Feet, specifically how a certain president can't:
The man in charge of the Nuclear Football simply cannot think and talk at the same time. Its like he has a broken task scheduler, a bad threading model, or there's not enough RAM, or random interrrupts are constantly firing. Perhaps he's computing Pi in the background; I just don't know. Even at the pace of that faux-Texas drawl, his brain simply cannot keep up with his mouth.
Vani and I listened to an old Fresh Air (available as podcasts!) interview with Al Gore the other day and one of the enjoyable things were just how he beautifully and quickly comes up with coherent and interesting answers even to slightly off the wall questions.
Remember that Bush is an ex chronic alcoholic. Apparently that can leave a person's brain pretty screwed up. So I'm going with the "random interrupts are constantly firing" explanation.