I guess it depends on your definition of backwards...there have been a number of advanced for their time civilizations in Africa.
Egypt/Kemet in North Africa which had grown from the Tasian, Badari, Amaranti, Gerza lands, plus trade partner/occasional subject/occasional ruler Nubia/Kush are examples of advanced civilizations in Africa. North Africa was also home of Carthage that managed to hold off the Roman empire for awhile and was a pretty remarkable country.
While the land of Punt is still speculative, I believe the general current consensus puts it in east Africa (Ethiopia/Eritrea area). That same area held Aksum, a major country in trade.
The Mali and Songhai empires were centered in west Africa. When the Mali empire was at its height, only the Mongol empire held more territory, from what I understand (although the Mongol Empire was still 9 times larger, so there was a big gap between #1 and #2). The Ghana empire predated and was eventually absorbed into the Mali empire.
Great Zimbabwe - in the area of Botswana, modern Zimbabwe and Mozambique - was another wealthy trade empire, with an impressive (now ruined) stone capitol city. You also get the Kongo Kingdom which was born of two earlier kingdoms, the Mbata kingdom and Mpemba Kasi.
So in general, while there were strong and advanced civilizations, they were laid waste by the things that lay waste to all countries: time, changes in circumstance (invasion, internal strife, loss of trade goods, environmental changes). Egypt, Nubia/Kush and Carthage were conquered by Rome (I believe, Nubia briefly conquered Egypt, and then Egypt and the Assyrians regained it, but the end seems to lie in Rome). The fate of Punt, like its location, is speculative, but by the later Kingdoms of Egypt was treated as an almost mythological land of plenty, so its decline so early in what we have records of probably leads to its mysterious fate.
Aksum & Zimbabwe seem to have suffered drops in trade that ultimately led to their collapse. Aksum splintered into other countries/kingdoms and part was taken control of by other countries.
As mentioned the Ghana empire collapsed, speculatively due to collapsing from Islamic invaders. Mali was greatly decreased by the rise of the Songhai empire. The Songhai empire had a succession crisis involving the king and his sons that weakened the nation. Morocco (founded from areas conquered from the Byzantine Empire by the Umayyad Caliphate) was able to conquered both empires.
The Kongo Kingdom was slowly converted after a Portuguese explorer found the kingdom and started spreading Christianity; it was subsequently destroyed in a series of wars with Portugal and the Dutch.
Modern Africa primarily doesn't really take shape until the colonial lands begin to gain independence (the last one in 1977!), and some areas have faired better post-colonialism than others, and their success or lack of happens for a variety of reasons. So there's not a single answer really as there is not a single reason or source for the development of the continent, but a tapestry of reasons woven through time and through various lands and peoples with differing desires and goals.
I always liked Georgia's war with North Carolina, personally for weird border changes.
