As the alien population grew the problem of tasty meals increased. People and aliens competing for the same food simply wasn’t working!
At the same time the aliens, who controlled the planet, decided they really loved having monkeys as pets yet only needed a minimal number of humans as menial servants.
At that point all unnecessary humans were carefully sorted and turned into premium monkey chow.
The monkeys would be well fed . .
And there would be a 90% decrease in competition for groceries worldwide.