I'm not a historian but my degree is in business and international development so I might be able to shed some light on your question. I won't go into it too much as there's such a long and rich history but I'm sure others will be able to add to what I say. Also, forgive me if I bounce around the subject. There's so much that I'd like to cover!

Firstly, you're absolutely right. Colonialism did have a large part to play in the unevenness of today's international system but there are lots of other forces at work that I feel have halted (or hindered) Africa's development.

Some African governments went to the World Bank and IMF for loans to repay existing loans that they had taken from commercial banks. These loans from the IMF and WB were known as Structural Adjustment Programmes (SAPs) and, in order to receive those loans, governments needed to adopt certain ("Western") policies, particularly liberal capitalist processes. The idea was to allow for Africa's markets to be liberalised, to encourage free international trade, and thus boost their economy. However, many would argue that this enabled developed countries to exploit parts of Africa for cheap resources, labour and land (it's interesting that you question whether Africa is rich in resources or not. They have oil, 30% of the world's mineral reserves, gas, precious metals - it's just that Africans don't tend to see any of it).

Through those SAPs, those more powerful, developed states, that make up the IMF and WB were effectively dictating parts of Africa's governance, whom they are allowed to trade with etc. So, it can be argued, how is a state ever going to fully develop and emerge as equal when they are still 'under the thumb' of the West? Furthermore, just because a liberal economy worked for one country's development, it doesn't necessarily mean it will work for another.

There is also a theory known as dependency theory, which would question how developing states are expected to catch up and equal the international playing field if the resources they need in order to do so are being used by developed countries for their own gain (note all the resources I listed earlier). Furthermore, a lot of African governments were/are corrupt. Some citizens never saw the money that the IMF and WB were funnelling into Africa. And then, interest was added on, more loans were needed, and more stipulations had to be adhered to.

There has also been continued humanitarian intervention (military) and aid-giving by developed nations in many African countries, despite their sovereignty. Some see the flow of monetary aid into Africa as leading to dependency on developed nations (much like with the loans from the IMF and WB, although, for me, there are some clear differences). In terms of humanitarian intervention, some see this as almost antagonistic (security promoting insecurity, negative freedom rather than positive, the West wading in without consideration of local ideologies, norms, wants and needs) or even as the West exerting its dominance on developing Africa, much like with the SAPs; all of this can halt or hinder development. Civil war has also made development difficult in parts of Africa and has increased security concerns (especially human security concerns that revolve around individual wellbeing). To go back to conflict, war also leads to damaged infrastructure and casualties, thus, your workforce decreases, and with it your country's productivity (disease and famine also present similar issues), so your economy suffers.

So there's a lot going on in some parts of Africa and probably a lot more that I could talk about but I think these are good starting points. I hope this was helpful.