Travel

On the average, most conventions are not local to many of the people that want to attend them. Which is to say they will probably have to Travel to get there. The travel my involve a medium to long car trip, or involve more extensive arrangements, such as train or aircraft travel. The farther the convention is from its target audience, the more this is a consideration. The potential difficulty and expense will tend to suppress membership.

A comprehensive look at the costs involved are at How To Penny-pinch a Worldcon.