The Rock beat Hogan twice. And I would hate to be watching a Wrestlemania main event being certain of who is going to win. I don't really like either of them to be honest. But we'll see how it all pans out.
Hogan was not the face of the company when Rock beat him twice, Rock was closer to that himself, even though he was just about done.
The reasons Cena vs Rock 2 will happen:
Rock is wrestling at WM 29
Cena "needs" (he doesn't) his win back on Rock, because he's the face of the WWE and nobody can ever look better than him, for any reason.
Vince doesn't want Punk facing Rock at WrestleMania, which is stupid and childish, because WM 29 will get as many buys as WM 28 if Rock is wrestling and it's built up, and Cena gets a fresher opponent to boot. I wouldn't be surprised if less people bought WM 29 just because Rock vs Cena already happened and the people got the result they WANTED the first time. With this one, everybody knows for a fact Cena's winning.