Plus, their methods of achieving the fission/fusion reaction are so inefficient, it leaves much of the radioactive material behind, to be dispersed as fallout. A close to 100% efficient reaction would consume all of the uranium/plutonium, and they could either use much, much less material to achieve the same effect, or they would be able to destroy much much larger areas of landmass.
Plus, their methods of achieving the fission/fusion reaction are so inefficient, it leaves much of the radioactive material behind, to be dispersed as fallout. A close to 100% efficient reaction would consume all of the uranium/plutonium, and they could either use much, much less material to achieve the same effect, or they would be able to destroy much much larger areas of landmass.