It's pretty complicated.
My guess would be that "resources" (staff, fields, etc.) limit the club's capacity to field multiple teams.
The shame of this is, who's "good" at 9 is often "OK" at 12 and "done" by 15. And who's a "B" player at 9 is often the best player by 16.
My advice to a parent whose child truly loves the game is to find a club that offers more options.
