Technology and Corporate Farming.
Families in those towns used to have tons of kids to do farm work and that is slowly but surely being replaced by corporate farms that either automate or bring in labor.
Turkey Valley is the best example. They won a 3A state track title in 1987 and 3A Baseball in 1985. Their most recent BEDS is 88, they're disappearing.
Rural schools are dying and the bedroom communities (Jesup, Dike, North Linn, Denver, etc) that have invested in their schools are maintaining or slightly growing and are the ones that are winning everything in Sports, Music and Speech.