Why is sickle-cell more common in african americans where their ancestors live in areas that had or have malaria?

Malaria resistance. Malaria is especially a killer of children. Those who carry the sickle trait - one normal and one s hemoglobin - have some resistance to this disease and this has caused persistence of this trait. Of course having both s and s causes severe disease and having either are not helpful in non-malaria regions. But it mutated in central africa and persists there in proportion to the malaria.