Ticketmaster has exclusivity on the infrastructure (venues) -- a rival service would need to create their own network of venues.

That's part of the issue but the bigger than venues, was the merger between LiveNation and Ticketmaster, that congress decided to approve.

LiveNation is the promoter of every major artist and tour. When that merger happened Ticketmaster gained the ability to basically block any Arena Size artist of going on tour if they weren't using Ticketmaster.