Cena is a poor worker with a shit load of charisma. He works his ass off for the company. He does countless interviews, is a nice guy backstage, and sells a crap load of merchandise. Paul Heyman wrote a pretty good article on why Cena is WWE's top guy
Regardless who he is facing he gets a reaction and makes people care.
http://www.thesun.co.uk/sol/homepage...John-Cena.html
Pretty good read. I really like Heyman's articles. I wish he had one every day.