I think it's because a lot of people are into the support bands they bring. Don't get me wrong, FIR has a big fanbase, but they went on tour with Ice Nine Kills, and those guys are getting some serious traction. They played a 500 cap room in New York (Gramercy Theatre) which got sold out. Now Ice Nine Kills is playing at Webster Hall which is about 2.5 times the size on their upcoming tour. I'm one of the fans who would go for Set It Off and not Falling In Reverse, 100%. However, because I'm there, it's going to make me look like I'm part of the Falling In Reverse fanbase. I would like Ronnie Radke to do some community service or something to make up for his terrible behavior. However, if someone recognizes me from a picture from their show, they will assume I was there for the headliner. TL;DR FIO is smart about support. That's why they sell their shows out.