"If you build it, they will come"
If you make an incredible rail park, people will show up!
Traveling to these events costs a good bit of money, and if there is not an economic incentive (cash, expensive boards) then there must be an emotional incentive (such as the opportunity to hit as awesome rail park).
Good Wakeboard and Wakeskate rails are hard to come by. If you build a lot of really large and fun rails, then people will have a reason to show up...
But Cash is still King. Last year, the Adventure Games in Deep Creek, MD was having some trouble getting some good riders to show up to their event, then they got a shop to pitch in $2,000 a cash purse. Once that was announced, people from as far as the state of Indiana showed up to compete. Here are some pictures I took from the event: http://www.wakeboardpittsburgh.com/2008/07/adventure-games-at-deep-creek-lake/