Some don't have a natural ability with horses, others don't have the courage. Some kids get too big. The first thing I do is try to discourage them, and half the time I do. I tell them they need to get an education, ask them how big their parents are. Too many kids waste their formative years, don't finish school, and if they don't make it as a jockey they have nowhere to turn.
Size is crucial.
Many kids spend their formative years trying to be jockeys, working in stables. Then by the time they're 18, they've outgrown the job. Anybody who made it was lucky they didn't grow. Bill Shoemaker, he had great hands. It's more dangerous than any sport.