As for why, he knows how to work a crowd.
Except, he has no idea how to work a crowd in a match. He has a formula he goes to in EVERY SINGLE match he wrestles in to get the crowd on his side. The shoulder blocks, spin out powerbomb, and five knuckle shuffle. Every single time. That's it. That is the extent of him working a crowd in a match.
The man is clueless as to the psychology aspect of putting together a wrestling match and relies on his opponent to handle that part. But in reality no one pays attention because until Cena hits his formula moves of doom, the whole crowd is chanting "Let's Go Cena", "Cena Sucks". So in reality I guess it doesn't matter anymore in wrestling. Watered down is acceptable now, I guess.