>>8078502 (OP)He was chosen to be a top guy early for obvious reasons, so he got away with things other people couldn't and he showed a lot of potential. He did good with his pushes but he eventually realized that he could coast without improving any further.
He has been on TV for 20 years straight at this point, largely doing the same thing so he's stale for people like me.
WWE also messed up his booking a bunch of times. They really could have gone further with The Legend Killer thing, but didn't. His month long run as "Stone Cold" Randy Orton when he won the title was awful. Him losing to Triple H in one of his first ever title defenses was an all-time bad booking decision. They never really established him as well as they could have as Cena's main rival. Legacy was a good idea, but WWE didn't really do their best with it. The IED angle was stupid and they had a chance to give him a new character when The Fiend burned him but chose not to soon after.