The highly successful UK Junior XC program vs. USA's nonexistantJunior XC program. Why?

Let me suggest that you take a longer term approach to a Junior Nationals like a 5 year or 10 year time frame. Even though our club has 35 Juniors, there's just one I know of that could participate in a Junior Nationals this year. If he's busy during that time, we'd send zero. I suspect other clubs would be in the same boat.

Why don't you consider a Junior XC soaring event as a way to prime the pump for a nationals that follow a year or two later? For example, every two years, we take our Juniors to Mifflin -any skill level, and bring several gliders and instructors. We fly 2-33's, 1-26's ASK-21's and Duo Discus and our Discus CS. We camp on the airport for several days and give the kids a chance to fly at a different airport, experience XC ridge soaring and so on.. They absolutely love it.

You could do the same. Arrange to have several XC instructors and aircraft come to a location (perhaps an East and a West event would be best due to distance). The intent of the session should be to stimulate interest in cross country flying for Juniors of varying skill levels. You can bracket the kids and have a few fly competitively while others learn XC. Start small, limit attendance -perhaps to those who have soloed or have a private pilot certificate and see how it goes. Do something crazy like vette them online using Condor ahead of time.

Keep in mind that whatever you do, it needs to be a parent friendly event. Younger Juniors don't have transportation and for those who do, today's parents aren't going to hand them the keys to SUV with a glider on the back and wave goodbye as they head cross country for the event. Kids are far more controlled and protected than you and I were at their age and even at age 18 and older. Giving parents a chance to help out or do something with their kids while there and making it a turnkey event with gliders to fly and instructors to help out will be important.

A year or two of an event like that should give you a much better idea of how to stimulate Junior XC interests.