People always cite exclusivity deals / monopoly power when it comes to Ticketmaster's dominance, but I also recall reading post-mortems about several failed competitors that indicate the problem Ticketmaster solves (massive spikey demand with strict guarantees on the seats selected) is quite technically challenging. I know, it doesn't seem like it would be that hard to solve, you're probably already thinking how you would do it. But you can't ignore that many others have tried and failed.