BoingBoing links to an interesting part of a Wikipedia article which presents a theory as to why the Pepsi Challenge failed:

In the 1980s, Pepsi's infamous "Pepsi Challenge" (El Reto Pepsi) campaign helped to virtually destroy the Pepsi brand in Peru, due in large part to the fact that consumers do not enjoy being told they're wrong. The campaign was quite simple actually: Tasting centers were set up in and around Lima where people could freely participate in a blind taste test between Pepsi and Coca-Cola. Attendees were presented with two covered bottles and two glasses, each bottle was opened and poured into its respective glass, whereupon the tester was asked to drink each and declare his or her favorite, but not before being asked which they preferred and drank regularly.

The campaign was a disaster, as one of three results came from the testing, all detrimental to Pepsi: 1) People were angered by the fact that they were "wrong" in their choice and abandoned Pepsi, switching to either Coca-Cola or Inca Kola; 2) Those who chose Coca-Cola over Pepsi either switched to or stayed with Coca-Cola; 3) Those who were ambivalent between them cemented their ambivalence and switched to Inca Kola.

Either that or Coke is just a lot better than Pepsi (living in Atlanta, I'm brainwashed to say that).