The correct answer is - poverty.
The developing world in general has a huge problem with poverty. Most of the developing world is in a very bad economic situation and is led by corrupt politicians, that contributes to the wealth and potential of the countries to be trapped by only few individuals, while the majority of the population is living in poverty, doesn't have access to basic things for life, the unemployment is huge, and the ones that work take just enough to be able to survive, and because of this there's huge amount of people that are trying to migrate to the more developed countries.