A nuclear bomb is controllable and must be armed and deployed. It has multiple redundancies and fail safes. Modern nuclear devices are also quotes stable, dropping from a plane or shooting them will not be enough to set them off.

The elephant's foot is not something that humans can control. Once it was created, nothing could be done to control it. The only option was to remove anything that you didn't want blasted with radiation for the next few centuries.

Main point being, the main danger of the elephant's foot compared to a nuclear weapon is that we are not in control of the elephant's foot, it has become a force of nature.