Because Iker has an eternal sticker of Holy Goalie attached to him and the only thing that can change it and cause him losing captaincy is that he becomes utter junk like Valdes used to be earlier in his career.
And on the flip side most of the football fans, who often happen to be morons, don't realize how much Valdes has improved and always think of him as a third post in Barca goal and the only reason that he has such good Zamora stats is that Barca dominate possession throughout 90% of their games.
I think Valdes is a more deserving keeper becomes he is a lot better GOALIE, but he won't get a credit for it as long as 1) he doesn't start for Spain, 2) Iker plays fairly solid or solid enough to maintain his reputation.