It's hard to know exactly where the vital organs are on a deer until you've field-dressed one and know firsthand. I think the main reason for heart/lung shot preference is the same as the main reason basketball players prefer layups and dunks over half-court shots. The size of the target area compared to how close you are to the deer is the largest with the heart/lung kill zone. If the heart/lung area is the "broad side of a barn," the head is "a small window adjacent to the broad side" and the spine is "a narrow white stripe painted on the facia boards of the broad side." The heart/lung area gives you the most room for error because of this. If you miss high, you'll still possibly hit the spine, whereas if you aim at the spine or head and miss high, you'll hit the dirt 1/2 mile behind the animal. If you miss left of the heart/lung aiming point, you still have a shot (believe it or not) at the heart. If you miss left of the head, you've shot an air-ball. If you miss right of heart/lung, you've got a shot at the liver, which bleeds heavily when hit. If you miss right of the head, you'll hit in the neck and drop the deer instantly, but this is perhaps the only good "miss" scenario, and most people will miss high and/or low before they miss left/right. Likewise, the spine is the only totally forgiving low-miss aiming point, but there's still good chance of hitting heart/lung even if you aim there and miss low.

Nate