The plant is built like that so spent fuel can be lifted out of the reactor and placed directly in the pool using one crane. A more secure spent-fuel pool would

The plant is built like that so spent fuel can be lifted out of the reactor and placed directly in the pool using one crane. A more secure spent-fuel pool would have required more elaborate fuel handling, thus a more expensive design. Also, when these plants were built I don't think they anticipated the spent-fuel pools would become long-term storage. There was the assumption that centralized nuclear waste disposal sites would be built and the pools would only store waste until it was safe to transfer.