The History Channel is a great source for word origins. I was watching a program about the old west. It seems that orphaned children from the East were taken by adoption guardians who would find homes for them with families. Often siblings were split up and children given to families that would then use them as slave labor. Boys were wanted more than girls and they spent time doing manual labor and chores. Even though the adoption agreements stated that the children were to be educated some didn’t receive any education past the fourth grade. The children were taken into towns and put on stages or platforms for men and women to examine. Thus, the term “up for adoption” was used as the children were raised up above the audience.
Because of this practice and the potential abuse of children, adoption agencies were established many of which still exist today.