As many have mentioned, it's all about guys like Berdych, Tsonga, and Delpo stepping up to knock them off.
Berdych has had several opportunities to knock off Rafa at slams in the last few years. He was steamrolling at the AO, then he just choked against Rafa in the Quarters. He had an opportunity to knock off Fed in 2009 at AO in the Quarters (he was up 2 sets to love but then choked).
Djokovic gave Tsonga 4 match points with the entire Chatrier behind Tsonga, and Tsonga still couldn't convert. Tsonga choked, then tanked in the 5th. Tsonga knocked off Federer at Wimby last year in stunning fashion, but couldn't follow it up against Nole.
Rafa well, I guess his aura at the French is bigger than his play. Almagro had a nice strategy against him but couldn't put the ball away when he needed to. He has had by far the easiest draw of the Big 4.
Murray's had several upon several opportunities to knock off Novak AND Rafa in the semis for years now. Mentally he kept collapsing. Although his mental game seems to be improving under Lendl. I really thought he was gonna beat Djokovic at the AO, but questioned if he had enough heart to beat Rafa in the Final should he have made it there.
Tsonga = All the potential there. Mentally, just not there. If he doesn't have that killer instinct now, he will never have it.
Berdych = All the power is there. Mentally? You have no idea who's going to show up.
Delpo = The game is there. Already beat a slam. He may never come back from the injury to where he was beforehand. Shame because he has a fun game to watch.
Almagro = Great all-around game. Fun to watch. Mentally just doesn't have what it takes to grind it out against the Big Dogs. Watched him today, and watched him against Nole at Indian Wells. All the strokes are there, just not mentally ready to go toe to toe for the distance.
Isner = After his serve breaks down because of leg fatigue, what does he have to offer?
I'm hoping Raonic is going to be one to not lay down and take a beating from them. I hope his loss to Fed at Madrid was a learning experience, and not something that will be stuck in his head the next time he plays one of the big 3.
Tomic I think is too laid back. He's still developing. Hope he develops a killer instinct.