There are many possible answers for this.
One way of thinking of this is to say that colonialism is what has caused the problems of poverty in Africa. We can say that the colonial powers set up economies that were meant only to extract resources from the countries and not to actually develop their economies. We can also say that the colonial powers did not educate the people enough to prepare them for self-government.
Another way to look at this is to say that African countries are impoverished because they lack infrastructure. They do not have good electricity and roads and water systems. These things scare away people who might invest in these countries and help them get out of poverty. Of course, these problems can be blamed on the colonial past as well.
Overall then, poverty in Africa appears to be caused by a lack of infrastructure and education and a lack of an industrial history upon which to build. Depending on your point of view, this can be blamed on the colonialists (and on modern "neocolonialism") or on the Africans themselves.