#3. A huge payday
Ultimately, every job in the world is about money regardless of the amount of passion and dedication that goes into it. There's no doubting Seth Rollins' passion and dedication for the business and there was no doubting the passion that CM Punk had - something that he admittedly lost over the years due to the grueling schedule and other circumstances.
However, big matches in wrestling always involve a big payday. Ask Brock Lesnar or anyone who feuds with him; pay-per-view paydays are known to be bigger and better.
Seth Rollins insists that there's only one stage that the fight can take place - WrestleMania, since it is the grandest stage of them all. This also means it's the grandest payday of them all.
Firstly, if they're going to have to convince Punk to do a match at WrestleMania, they're going to have to come up with a figure that has a lot of zeros. Given how Vince McMahon basically "throws the vault" at Brock Lesnar, it's a no-brainer that he can do the same with Punk, especially given that he would make a whole lot of money for the company.