Because they are out of people to feed him. I mean do you want more Orton/Cena, Sheamus/Cena etc etc etc.
Cena is played out and has been. I think the way they played his open challenges at least do not squash the mid carders as they are coming for a chance they otherwise wouldn't get.
I will say this, Cena is worse than Hogan ever thought of and that's crazy to say, but at least Hogan didn't squash someone every show. He drew for the house shows and was the ME at PPV type shows but he wasn't on every single week burying the whole roster.