For many years, the church has registered and paid for all boys, ages 12 through 18 in the BSA.  As long as a young man was a member of the church (and this included those who weren't active in church) he would be registered.  The church was spending lots of money paying registration fees for boys who weren't involved in scouts or church. A waste of valuable resources for sure.

If a young man is ever interested in Scouting it is usually early on, between the ages of 12-14.  A commitment to earning the Eagle comes usually at that age.  When a boy reaches high school, his interests change.  No longer is scouting a priority, or something "cool", as he is now involved in many other extracurricular activity including sports, and girls.

IMO the change was made to address the waning interest of boys 14-18.  Looking at the link shared above, it's still the scouting program without being labeled a BSA program and without the associated fees, uniforms, etc that BSA entails.