@Henrik phil,
This is a highly multifactorial problem, and it's not solely the blame of one group or another.
I'd highlight a few major impediments to development in Africa
1) Geographic isolation. Africa has the smallest ratio of shoreline to landmass of any continent. This means that a MUCH higher proportion of its people live far away from the coast than, say, Europe, which is much smaller physically but has far more coastline. To make matters worse, even to this day the Sahara is an immense barrier (trust me, the first time I flew over it from north to south it took around 6 hours to get from the Mediterranean to the southern fringes of the Sahel).
This means that African societies have been relatively
unlikely to be able to share in many of the societies like Egypt, Greece, Rome, Persia, Babylon, etc that developed in the near / middle east. There WERE some great societies in ancient Africa, most famously Axum (Ethiopia), the empires of Ghana / Mali / Songhai (Mali), Kano (Nigeria), and Great Zimbabwe. But these were, in the end, unsustained.
When the Suez Canal was built in the late 19th century, European seafarers began to completely bypass Africa, so colonial cities (like Calcutta, Bangkok, Singapore) never really developed in Africa. The colonial cities I've personally been to, like Saint-Louis (Senegal) and Banjul (Gambia) are cute relics.
2) Difficulties with agriculture. To have population growth, urbanization, and food surpluses, you need sedentary populations and agriculture. The problem is that sedentary populations get ravaged by malaria and sleeping sickness. Irrigation has been a huge contributor to mosquito growth. And believe it or not elephants were a major impediment to developing agriculture because they were nearly impossible to keep out of farms until recent history.
3) Disease. Africa has an inordinate burden of disease. Malaria alone consumes 30% of the health budget in Africa and kills a million African children under 5 every year. The combination of recurrent malaria, recurrent gastroenteritis, chronic parasitic disease (esp hookworm), and the consequent malnutrition caused by these infections, greatly impedes the physical and cognitive development of children and therefore their productivity as adults.
4) Slavery. The European slave trade exported around 20 million Africans from the continent (half of whom died en route to the New World). But there was also
in situ forced labor, i.e. colonizing powers who enslaved and brutalized the indigenous population. Think about the Boer War, the genocide of the Hereros, and the catastrophic reign of King Leopold II (Belgium) who is blamed for reducing the population of the Congo by 10 million people. The Arabic, trans-Saharan, and indigenous slave trades were a trifle compared with this massive depopulation. In addition to robbing Africa (esp West Africa) of millions, it also turned Africa into a slave economy (often slaves were exchanged for arms), and offered no chance of stable societies ever developing.
5) Diplomatic manipulation: Many wars, atrocities, and puppet regimes have existed in Africa since the end of colonialism in the 1950s. A lot of this had to do with communist vs non-communist alignment. Some of the most catastrophic examples were Angola, Congo, Mozambique, and Ethiopia. The Congo's current horrendous problems are directly traceable to the assassination of Patrice Lumumba more than 40 years ago, which the CIA (but especially Belgium) had complicity in.
6) Debt. Africa has been saddled with tremendous debt because of development loans. Payment on the interest of these loans exceeds the gross domestic product in many cases.
7) Economic manipulation. African raw materials are bought on the cheap, then resold to Africa in more expensive processed form. A good example is cocoa from Ghana and Cote d'Ivoire. Nestle has virtual monopolies in much of Africa on the sale of processed cocoa products. Cocoa and coffee plantations in this region have a big problem with child labor and even child slavery -- it's lucrative to the plantation owners to sell their products, but the reimbursement never spreads out to the population. Worse yet are the protectionist policies of America (and probably Europe, I'm just less familiar) with some exports. American subsidies of cotton and rice, combined with the efficiency of industrial farming, means that American cotton and rice can be sold in Africa for far less than the local product. This has put cotton farmers in Mali (one of the world's poorest countries) out of work, and has nearly destroyed the Ghanaian rice industry.
8) Horrible governments. Corruption is rife. But what can you do, the populace isn't empowered at all -- they're barely literate in most places.
9) Neglect. Africa just isn't a strategic priority for the rest of the world. It's becoming so as the Chad-Cameroon pipeline starts cranking out oil, and with oil reserves in Equatorial Guinea, Nigeria, and probably elsewhere in central Africa. But no one is really making great pains to create a Somali government, to reconstruct Angola / Sierra Leone / Liberia after their wars, to broker stability in Zimbabwe, to bring peace to Burundi and Sudan, etc. Just not on the world's priority list.