It's really a combination of factors, though I really believe the environment played a huge part since after all, it influence whatever culture is born in that place.
Poor countries have weak people, acting like mercenaries for any money, unable to think bigger, to think for the common convenience (thinking only in themselves). This results in corrupt leaders, dirty cities, misery and chaos.
Another important factor is demographics. In rich countries like Japan and Germany, the couples normally have only one or two children. But in the developed world (especially South Asia and Sub Saharan Africa), they produce children in the dozens.
I believe I heard some people point out that without the negative effects of the Black Death, Europe today would be like sub-Saharan Africa. It did certainly have certain undeniable effects. It depopulated large tracts of land, forcing feudal lords to raise wages or make concessions, eventually spelling the end for the feudal system, etc.
You will also find about 20 nations of the world that refused IMF membership. They are the poorest countries of the world, with plague, disease, famine, pestilence, and war destroying them. Note that things have changed since 19966, so you won't find the same member and non-member nations in the IMF.
What does that all mean?
Globalist conspiracy! How dare they!