It does seem common in USAG gyms to just test out of level 6 and go straight to 7. This would account for the large drop from 5-6 but the stability from 6-7
Major factors for drop outs are usually age, this can cause.
1. School pressures increase, there is more homework and more expectation to be involved in extra activities at school.
2. Social lives change, when kids are little and they go the the gym it is an outing. If they were not at the gym they would be at home so it's their chance to go out and be with friends. When they get older their freedom increases and if they weren't at gymnastics they would be out with friends and doing things.
3. Interests change, just as when kids get older they lose interest in dolls the same goes for many childhood interests.
4. Peer pressure, they feel if they are always saying no to parties and things they will be social outcasts which can be a fate worse that death for these kids.
5. changing to middle school or high school often means they must travel further to school, en extra half and hour to and from school is actually an extra 10 hours a week of less time they have.
6. Boys, need I say more.
7. Part time jobs, friends are getting jobs and they can buy the latest gadgets and clothes and phones.
8. Self consciousness, many kids hit level 6 about the same time they hit puberty and they are no longer keen to prance around in nothing but a leotard.
9. Parent pressure to do something "real" with their lives. As kids head towards middle and high school, there is pressure to be preparing for a career, studying a lot and so on.