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This is a very perceptive question. The answer is partly about anatomy, and partly about physics.
Lumbar vertebrae are quite a bit more massive than cervical vertebrae. The thinnest and lightest vertebral bone is the atlas, which is the topmost bone of the spine, the one the skull rests directly on. As you move down the spinal column, each successive bone is a bit heavier than the one above. This is because each bone has to bear the weight of all the bones above it, as well as the weight of the skull. Since the bones nearest the top are the thinnest, and are also the least well supported by surrounding musculature, they are the most likely to suffer fractures.
Conversely, the lumbar vertebrae, being large and strong, don't often break; they are more likely to suffer damage to the intervertebral discs. A ruptured or herniated disc is more likely in the lower part of the spine partly because the discs there are bigger, and partly because this area of the spine is doing the heavy work of holding up much of the weight of the torso, head, and arms.
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