He lost his momentum after his match with John Cena to decide the Undisputed Champion. His entire platform and the sole reason for his IWC success was his anti-establishment angle which lasted all of 2 months.
The shoot was terrific, the theme change was even better, and suddenly everything the IWC has been complaining about was being voiced by quite possibly the most underrated superstar at the time on WWE's biggest show live, week after week.
Alright, so come Summerslam everything that needed to be said had been said, and CM Punk's victory would put him over until Kane was done wasting Cena's time.
Where are we now and where have we been since? The focus remains on Cena, and the WWE is running just the way it used to. Nothing has changed, there was no "revolution", and CM Punk has let down everyone that vouched for him in hopes for a change in times. The focus is still on Cena, as well. The only difference being that CM Punk has now solidified his status as a main eventer for at least the next year and a half.
Why not longer? How could CM Punk ever be forgotten again after what he did? The Miz.