Mania 28 had the highest Mania PPV buyrate of all time, so that probably had a huge impact on why WWE decided to do Rock/Cena again. Obviously, by the way they were touting their first match at Mania 28 as "Once in a Lifetime", there were no plans to do the match again. Once the numbers came out, they likely thought that adding the title would result in similar numbers for 29. And they weren't that far off to be honest. I think Mania 29 was something like the 5th or 6th highest of all time.

I guess I can understand Punk's gripe, he had a legitimate claim to the main event after carrying the title for 434 days, but it's not like he was being left out. He did face Taker and was apparently scheduled to face HHH at 30. There was a lot more that went into Punk's leaving, the guy was pretty much miserable near the end and I doubt a Mania main event would have changed that. I remember him saying in an interview that he was more than unmotivated in his program with Taker, I think he actually said he didn't care at that point which is when he realized that he wasn't going to be around much longer.

Had he stayed around, he probably would have gotten his main event, than again, with the emergence of Reigns and Lesnar, he might not have. Basically, Punk wasn't a WWE guy and I doubt Vince found him reliable enough to carry the brand. Ultimately, he was right.