Serizawa: The only way to heal our wounds is to make peace with the demons who opened them.
Dr. Stanton: Oh, yeah, sure. Let's bring him in for a beer.
Mark Russell: No. This time we join the fight.
Mark Russell: How many of these things are there?
Madison Russell: One, two.
Dr. Ishiro Serizawa: Seventeen - And counting.
Chief Warrant Officer Barnes: Sometimes I think this is Godzilla's world. We just live in it.
Elizabeth Ludlow: Damn right.
Dr. Emma Russell: Long live the king.
Jonah Alan: We opened Pandora's box. And there's no closing it now.
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.