That has a lot to do with sports infrastructure, corruption and even wealth in general. There's way less sports centres for talents let alone on a top level to instruct and develop young potentially promising players in most of Africas countries and even the well known african football/soccer countries are rife with corruption and unnecessary involvement of politics aka government since decades which dampers their potential to do well in World Cup.
Despite these circumstances there's still a lot of great football players produced that came from african continent and racism plays a part into why only George Weah got a Balon Dor so far.