This is just speculation on my part.
But of all the theories on matches happening at Wrestlemania 29, this is one I haven't seen suggested. If you remember, the WM29 poster had Cena, Brock and Rock on it, and no one else. Obviously, this was just WWE putting their three biggest draws on the poster, but what are the odds that this could end up being the WWE Title match? This, and Taker/Punk in a Respect vs Streak match, would be one hell of a double main event. The only negative things about it is angle wise, why would Brock want the WWE Title? And also, how would Punk feel about losing the belt to Rock, only for Paul Heyman's other guy to get to be the one to fight for the belt? (That second one probably has a plausible answer - Punk says beating Taker's streak means more than keeping the world title for another day since he already proved himself a great world champion, and could also claim Rock beating him means little since he still held the belt longer than Rock ever did.) Also, no one really comes out a bigger star out of any of these matches (there's always Ryback winning the WHC, though, and whatever they decide to do with Dean Ambrose/The Shield), but WWE are obviously just interested in big marquee matches, and I think this is a great main event, putting a huge Hollywood star, UFC's biggest draw ever and their own top guy in the main event of the card.