Because someone thought that booking a show on the weekend of July 4th was a good idea. Usually, I'd agree but for some problems.
One, it's on a Sunday. The afternoon would be a good time to see these bands finish up, but most people are going to be coming home from wherever they've been at for the 4th of July holiday. Also, most people have to work the next day. I know I do.
Two, this show is during the same weekend as the Ozzfest (San Diego is on Thusday, Glen Helen is on Saturday). Anyone not dead from heat stroke at these shows will probably be home recovering.
I really think attendance will be low by the time the Maidens hit the stage.