One factor not mentioned in adoption is the birth father. He has constitutional rights. A “crisis” pregnancy may involve a horrible, sometimes fear based relationship with the father or no relationship at all. In the process of placing the baby for adoption, the mother may fear that he will step in and get custody.
I don't know how "constitutional" his rights are, but one of numerous reasons why women choose abortion is because they don't want to be permanently associated with their babies' fathers.
A woman is told that she alone has the right to destroy her baby, but if she keeps it, there's no guarantee she won't lose custody to the father, or have to permit him to visit, or even pay him child support. Once she (and perhaps her advising friends and family) realize that the only way to avoid making "him" a permanent presence in their lives is to abort, they find killing convenient.
Or she is afraid that he will deny the adoption without taking the baby, leaving her with the child she doesn’t want.