The Development Gap
Historical Reasons
Around 1000 years ago, Asia was the most wealthy continent in the world whereas the continents Africa and Europe were poorer (they were equally wealthy to each other.)
Then the industrial revolution occurred in Britain and Europe and North America (Brits had already settled there) became much wealthier.
The wealthy countries stopped trading with poorer countries fairly and started taking over poorer countries. Only now how the colonies been claimed back.
Geographical Reasons
Some countries have a bad location in the world with: High tectonic activity; extreme weather; droughts/floods. This stops the countries from developing as efficiently.
Other countries have a great location in the world with: perfect weather and climate; low tectonic activity; large amount of oil; easy access to trades. This greatly promotes development in these countries.
Health and Education
Some countries have many bright children who are willing to learn but just don't have the facilities to do so (no schools/books.) As a result, they cannot help the countries develop and become better.
Likewise some countries are unable to supply good healthcare and, as a result, some people are constantly sick (cannot work and help country develop.)
If you are poor, you will spend most of your life looking for food, water and shelter to survive the day, rather than helping the country develop. It is a feedback loop, in which poor countries create desperate people who in turn, focus all there life on surviving, meaning the country remains poor.
Conflict and Corruption
Some of the poorer countries have a hard time developing due to constant war (people have to hide and survive rather than work.) Likewise the money spent on war equipment (tanks) also costs a fortune and is also why the country is poor (these wars are waged for a variety of reasons: religion; corruption).
Also, greedy political leaders steal money from the poor.
High Risk Exports
Some poor countries rely on a few exports to supply them with money. This is risky because if the demand in the other countries drops, then the producing country may find itself in a lot of trouble.
For example, if a country exports exclusively tobacco and develops thanks to the tobacco sales. Say scientists find out smoking tobacco now will kill you in 1 year, sales for tobacco drop and, the exporting country stops generating income. As a result, this country doesn't have any money for developement.
Lack of Industry
Big companies of products such as chocolate want to earn the largest profit possible, meaning they buy cheap, and sell expensive. As they keep cutting down costs on cocoa powder, and keep increasing costs for chocolate, both you and developing countries (who sell cocoa power) are finding yourselves in a loss. This, similarly to "High Risk Exports" means the development in these low income countries is reduced.