I feel like most of the smarks know why Punk left (jobbing to H's) and general discontent with the way wwe books. Punk has kinda become the symbol in a way for the fans who are unhappy with the way the product is. But putting all of those feelings into a chant would be really long and impossible to do, thus
CM PUNK CM PUNK CM PUNK
I'm really fucked up right now, so I may not have explained that very well but I assure you the logic is there
Plus I think that most people genuinely want punk back and think that by chanting it will either a) make punk want to come back somehow...i dunno b) scare wwe into giving him what he wants to come back, which isn't happening.
Although I am secretly holding out for a suprise return at Mania to screw over bryan and become Corporate Machine Punk
I'd really like to see a cena heel turn. He's been stale for years now. He was never very good in the first place (wont go into that), but he would be able to bring the heat well if he started insulting the crowd.
Most of the people chanting for Punk probably don't even know why, but are going along with it in the same way people go along with chanting for Cole and King when they're really bored. All it takes is one smarky crowd to get a chant loud enough on TV that it will show up in other cities just because.
Cena is the sacrificial lamb, or something. Hence why his arms were tied up it's like being on a cross without it being a cross and offending religious folks.