This disease hasn't dissapeared because the recessive allele
that causes it makes carriers more resistant to malaria
. Hence, this increases the chance of a carrier surviving even though some of their children will die young from the disease.
If two carriers have children, there is a 1/4 chance the child will be a sufferer, a 1/2 chance they will be a carrier, and a 1/4 chance they will be normal.