1. Because he's the only full-time guy on the roster right now who isn't comedy bullshit. The fans are sick and tired of comedy bullshit. Other than places like South Carolina, where the crowds are dumb as hell, cheer for Cena, and still think pro wrestling is real, everywhere else is sick to death of the entire show being completely overrun with comedy garbage. Every single segment now is filled with it. Even the Shield right now, who should be intense and awesome and fantastic, are stuck feuding with Sheamus' goofy comedy ass, and Big Show who comes out and gives promos about how back in the day he didn't used to have a hand but now he does so he can totes win now you guys. It's all garbage. All of it. And then Punk comes out, and for 15 minutes or so, G-rated kiddie comedy hour sports entertainment tripe becomes pro wrestling again. And the fans fucking LOVE THAT SHIT. Even the casuals, who don't really understand WHY they love it, still love it. So they cheer Punk.
2. That segment, and the ruse he pulled on Taker, was brilliant. He pulled one over on Taker, got his one up on him, and used some awesome nostalgia to do it (discounting the Bearer heel heat move which, tbh, Taker no-sold pretty badly, which may have contributed to the lack of heat for the segment). A lot of the cheers for that shit were straight up respect for a job well done. Like good job yo, you got yours, well done, see you on Sunday for your ass-kicking.