Why are the most polluted cities in the world located in developing nations?

Because of the bad planning.

The industrialization, urbanization intensified worldwide, but this occurred first in the central countries, which undertook a classic industrial process in the eighteenth century. And finally, from the twentieth century, in the peripheral countries, who were late to know their respective productive developments.

The formation and growth of cities, when they occur at an accelerated rate - as in today's underdeveloped countries - generate a number of social and environmental problems in their geographical spaces. A large number of people living in a structurally unprepared environment, together with pollution from factories and vehicles, result in, among other factors, rising pollution rates.

the anwser is a) to prevent military conflict