The main reason(s) why industrialized countries are economically prosperous and underdeveloped countries?

In Jared Diamond's "Guns, Germs, and Steel" he theorizes that economically prosperous cultures got ahead hundreds of generations ago due to the physical geography. For example, the easier it was to eke a living out of a particular environment, the more "free time," so to speak, a culture had to devote to things other than merely surviving- food, shelter, safety. Also, the easier it was to travel, the more easily cultures traded and shared knowledge with their neighbors, contributing to a greater wealth of shared information and technology.