The difference between the American route and the African route

Just wanna share an observation which has bugged me for a while. I can't but wonder why foreign airlines operating in Nigeria ( BA, KLM, Lufthansa, etc ) fly the Nigerian route with aircrafts they dare not venture into the North American airspace with. C'mon, the Nigeria route is one of their most lucrative so whats the problem with bringing us newer aircrafts like they do on their European and North American routes ?