They can only afford to accomodate so many riders - both in a financial and a spacial way. For instance, if a ride has an overnight component, many times rooms will be provided in college dorms - there are only so many of these to go around.

Financially, every rider has an inherent cost to the organizers. Each rider must have paperwork, each rider must have meals, etc. Costs grow almost exponentially while income grows linearly...

The way I think most mature rides handle is like this - figure out the maximum number of riders you can accomodate that results in as little a percentage of overhead as possible. Figure out the minimums required to raise the most money yet still fill the ride.