It's because poverty causes both overpopulation and hunger. Poor places tend to be agrarian and pre-industrial, and under those circumstances, people have a lot of kids to help tend to the fields and take care of them when they're old.
@BlueVeins and a shame (as displayed below) that this situation is wrapped up as "breeding like rabbits and they're too stupid to use protection". That obviously isn't the case but it suits horrid stereotypes
