Based on my research into why folks don't do something in model aviation (surveys) what keeps people from attending things is typically time and money. There will be some other one off things that you can try to address or not depending on your inclination. I mean, the organizers of the IHLGF could ask me why I haven't been back since 2001 and the answer would be pretty simple: time and money.
One interesting thing would be to look at people from outside of say an 8 hour drive of Muncie that do attend contests that they have to travel 8+ hours to but don't attend the Nats. What would be interesting would be to see what it would take to swing them to attending the Nats over what they currently attend (team selections, The Bruce, Polecat, IHLGF, SWC, Visalia, F3J Tour stuff, World Champs, etc). Based on what I have observed and heard from folks that fit that description I have some specific thoughts on that that I shared a bit earlier in the thread.
Ryan