At one time it was believed that the reason for their underdevelopment was because they were exploited by the developed world – which was certainly true. And once this ended, they would become developed all in their own (with maybe a little help from some generous benefactors) – which did not happen. And in my opinion – will not happen. For reasons I will go into.
The developed world became developed – became affluent – because it had acquired special skills. Chief of which was the discovery of absolute time and space - which is necessary to do any kind of planning. A discovery that differentiated Northern Europe from Southern Europe – at the time of the Reformation.
One of the first Sociologists, Max Weber, did a survey in 1916 – and discovered something that had not been noticed before. There were rich countries in the West and their were poor countries. And the rich countries were Protestant and and poor countries were Catholic!
He attributed this, correctly to the Industrial Revolution – which happened mostly in the North. But did not understand Science well enough to understand why. The North understood much more about Reality – and was able to turn this into their advantage – with a whole series of technologies.
Latin America retained its Late Medieval culture – and did not progress at all. Because, basically, it did not want to. As far as it was concerned. it already had a fine culture (and a perfect religion) – and didn’t need anything better.
Even countries which were not Christian – as in much of Africa – have much the same problem. They do have the skills necessary to develop themselves. And they had no way of developing these skills – as far as most of the population were concerned – and it was necessary for most people, not just an elite few – to have them.
So far I have been talking of the skills necessary for an Industrial society. But an even more important skills are required for Democracy. A very tricky skill set that few nations develop. Including America at this time – which just barely has a democracy.
Developing nations are not likely to develop these on their own. And even developed countries can sometime lose them – as Germany did during the Nazi era.