Even in countries with relatively similar geographical conditions, both in terms of weather and resources, politics and corruption can make a huge difference. Again, consider Saudi Arabia and Sudan. Where Saudi Arabia has a strong ruling monarchy and government that has allowed stable growth for a long time Sudan is emerging from a civil war that has recently split it into North Sudan and South Sudan. Neither yet has stable government and money and resources are wasted through lack of leadership as well as the costs of war.
Corruption is also an issue as many sub-Saharan African nations have un-elected governments that are run for the profit of those in charge rather than for the good of the people. Sudan is an example of this system, as is Zimbabwe where the ‘President’, Robert Mugabe, has kicked people off their land to give it to his own men, sponsored the murder of those who oppose him and forced workers to grow food to sell abroad while his own people starve. He lost the last election but remained in power after accusing the system of being tampered with.