It's the size of the venues. Very few large venues don't strictly use Ticketmaster, and the big acts are contractually obligated to perform at large scale for revenues for their management companies. For large venues, Ticketmaster has become basically a monopoly, but regulation efforts tend to focus on their surcharges and fees, rather than just how damn many venues they control. One of the biggest draws in the industry, U2, are performing concerts at a venue in Las Vegas this fall without their drummer, which is nuts for such a tight-knit band that have been together since they were teenagers, because he needs surgery and recovery time but they are contractually obligated through Ticketmaster/LiveNation to perform. They were managed for a decade by LiveNation/Ticketmaster until recently calling it quits, just because they couldn't stand the company anymore. But they are still bound to them for reserving exclusive venues and ticket sales, just because no one else operates at Ticketmaster/LiveNation's scale