Apparently he's out of the cup injured.
That, and even if he wasn't injured, you have to realize that to the team, Iker is a captain whom every really likes and listens to. Him assuming responsibility for the last match and being an experienced captain I'd give him once more chance. It is his last world cup, and he can guide the team one last time. Look what happened to Real Madrid in the big matches in the league, the only competition Casillas didn't participate in: they got demolished. Big teams, even big players, need their captain. Diego Lopez is a good outfit and all but he commands little respect and cannot guide the team, while I think De Gea is absolutely outstanding, the likes of Ramos, Xavi, Busi, all world champions won't listen to him. Had Puyol been in the team and Casillas can leave the captaincy to him, I'd be fine with it but who will captain this side? See the comments made by Sirigu who was fantastic for Italy against England, he respectfully said it best: Buffon is more than just a player. Same goes for Iker.
What kept Spain at the top was a very diplomatic political policy revolved around harmony and skill. More skill of course but a lot of harmony too. Why is Del Bosque so intrusting of new comers? He already included one player that bitched, Silva and he needs to maintain that harmonic balance. Also, Iker sacrificed himself for this Spain side, these specific players so they all respect him and want him in the team. Dropping the captain isn't dropping just an off form player, when the coach deploys his players on the pitch, that is it, the captain is the coach, and if you don't have someone whom the players listen to, its done, just like Real Madrid was in the league. It doesn't help that Pique has been terrible for two years, and in my opinion has turned into an egomaniac, add to that Ramos who has his own issues, yo think De Gea would set them straight? Even Alba who is a hot head, Busi and Xabi are also a bit loose in the head, add to it Costa. Who do you think can keep these guys in check? Man, there is a lot at stake here to introduce a young gun like De Gea in place of Casillas.
Finally, assess the goals exactly, Iker wasn't spectacular but he wasn't totally at fault until the 4th. He saved Sniejder's shot with one handed super reflex. Spain were playing well until Pique kept Robin Van Persie onside, the whipped in ball was met with a crazy header than no one expected. Some people said Iker was out of position, he wasn't. Goal keepers do not hug their lines, they are ready to charge out of the goal and Casillas was standing where he should have been. The second goal came from an outstanding Robben control, which turned Pique inside out and Iker had no chance. The third goal is what people blame on Casillas but remember Iker was blocked by Robin Van Persie, normally those are called in favor of the goal keeper (see Croatia). It is a continental practice to protect the goal keeper. Having said that, Iker should've done better, and IMO De Gea would've had the height for it. The fourth breaks my heart. The fifth, how is it his fault? Then he gets up and produces two outstanding saves. He wasn't spectacular neither in reflexes or command but that was a huge implosion by Spain's midfield and tactically lies at Silva's feet who doesn't fit this formation because Spain and BArcelona are used to having players hold the ball up, AS LONG AS POSSIBLE AS FAR UP AS POSSIBLE. Silva and Costa didn't do that so it was wave after wave of attacked on the defense.