>>13425971'The E' signed him because he was already a huge draw dumbass cultist, he could show up any day and he would put thousands of asses in the seats, at the time Cena was probably the only other guy with the same kind of popularity and even that's debatable which is why his signing was a big deal.
WWE is just another promotion even if it's the big one, he had a shit run there for many reasons but his performances and numbers both before signing and after leaving have been stellar.