Other mistake: When Godzilla emerges from the sea towards the end of the movie, what is he standing on? You can see the submarine at the side of him so the water must be reasonably deep. Then he just swims away. (01:37:15)
Continuity mistake: When Monster Zero is standing victorious over the Mexican city, a traffic light changes from red to yellow.
Revealing mistake: The signal sweeps shown on the 'orca' device and on other screens throughout the movie are all identical.
Answer: I don't remember if either Godzilla 2014 or Godzilla: King of the Monsters directly said that this was the case, but the MUTOs were feasting on nuclear radiation and warheads, meaning that their presence and deaths in the area would most likely unleash a ton of radiation. Radiation can make entire areas uninhabitable for years.