Attendance at Amiga shows in the U.S. has been steadily dropping for the past 6 years or so.
Typical marketing includes web site posts and mentions at user's groups in addition to interested folks being on the show mailing lists. Given Amigans come far and wide when they come it's a tough thing to market to.
The sad truth is that folks just aren't interested enough to show as they used to be. It's tough for the show promoters because they have to put up a deposit and risk low attendance and usually have to pony up an amount on the back end no matter what. It's not like they set out to have low attendance.
