Cena choosing Bryan, because Cena's endorsement is what began Bryan's rise as a main-eventer. Cena's endorsement, in terms of becoming a Main Eventer, is perhaps bigger then winning any match can be. He endorsed Punk in 2011 and demanded Vince bring him back after he fired him, and it was the match he had with Cena following the pipebomb and Cena's endorsement that completed his legitimacy as a main eventer, but if Cena doesn't stand up to Vince and endorsed Punk as the 'best challenger', Punk doesn't become "Best In The World".
Same thing with Bryan here.