Many viewers of the show have questioned why Mary and Adam didn’t adopt the Cooper children during the later seasons of Little House on the Prairie.
One of the primary reasons for this is that Melissa Sue Anderson, who had portrayed Mary Ingalls on the show, was leaving the television series at the end of the season that introduced the Sanderson kids. She no longer felt the role was fun and that her character kept getting hit with repeated tragedies as well as limitations of portraying a character that was blind for the majority of the show’s run.
With Charles Ingalls adopting the kids, it kept his character centric as a father, as both Mary and Laura had left the house by this point in the series, and neither the Carrie nor Grace characters were being developed.