He lost to the Rock.
Remember how everyone just KNEW Cena was going to win at Wrestlemania? Myself included mind you.
Yeah, he lost. Clean. So why not again? From all accounts (hell even Colt Cabana and Punk went "yeah, used to think he was a dick, but then he turned out to be a nice guy") he seems interested in pushing other talent (or helping young talent, once again, according to Punk and Colt he's now working with Trent Baretta to help him get somewhere), but WWE is too protective of him.
I can see Cena wanting to help push a guy like Bryan. He did help Punk get over last year.
I think the problem is that although WWE is waaaay overprotective of their stars (you can see th is in the way they have Punk come out on top ever since he became the #2 guy in the company last year), Cena doesn't really seem to care about making it work... or maybe he just doesn't have the ability to.
Punk still manages to pull off good matches even when he's being booked as superman. Austin won like 95% of his matches during the height of the attitude era but it never felt like it because he had the ability to make it engaging, and he usually won in unique ways (Vince is referee? Knock him out with the stunner, grab his hand, and slam it to the mat 3 times. Cage match with Vince, and Big Show is interfering/debuting on behalf of Vince? Have one of the sides of the cage detach when austin is thrown into it by show, and have him drop to the floor, escaping with victory by a miracle, but bloodied and beaten.)
Cena gets a cinderblock thrown over his head or DDT'd on concrete in a 4 on 1 handicap match and he goes on a rampage 2 minutes later for a victory. It's way, way ,way too blatant and supermanish. Same with his promos. They're just TOO corny.
From all accounts, Cena is an amazing guy that gets along with everyone else backstage, and his work ethic is second to none. HOWEVER, he also seems to be just as much of a yes man as Kane, perhaps even moreso, since he has Vince's ear in a way that Kane could never dream of.
Every time Cena feuds with someone, he buries them. Sometimes it's in the match (dropping chairs on wade, no-selling head shots on concrete). Sometimes it's in the buildup to the match ("If you fail to beat me, Show, you'll be the phony everyone always said you were" *beats Show at the PPV*). Sometimes it's after the match, especially when Cena loses ("Well, I lost, but I gave it my all and WE TORE THE ROOF OFF THE PLACE WOO YEAH!" That kind of it-is-what-it-is attitude KILLS crowd heat. Or when he was supposed to be carried off after beating Lesnar but instead gave a promo, no-selling the brutal beatdown he was supposed to have received.)
I think Cena is a good guy, with good intentions, and a noble heart, but he kind of kills any buildup other wrestlers create for themselves in promos, and he fails to tell a believable story in the ring, so when he wins, it makes his opponent look incredibly weak. That's all on him, not Vince. A superman face gimmick CAN work, but not if you come off as phony and fake about it.