If slams were played best out of 3, Roger would be a co-favorite in all of them but the FO. It's pretty telling how he's able to roll back the years so often in Masters 1000 events for the past 3 years, and have these vintage performances.
It's not that he's not able to do it in slams, he does it at times, but the task is so much difficult when playing best out of 5 set tennis. Wimbledon 2012, to me, will always remain one of his most remarkable achievements for precisely that reason. He came close to replicating that this year, but again, the challenge of doing it over 5 sets against younger, fitter guys (I'm referring to the obvious three guys here, or even some big hitters) is just that much more daunting.