Krakauer ignores his inner voice mainly because it is a habitual, unreliable alarm that cries wolf. He compares it to "Chicken Little," noting that it "was screaming that I was about to die, but it did that almost every time I laced up my climbing boots." In other words, the sensation is a frequent, recurring fear response rather than a dependable signal of real danger.
Secondarily, practical considerations push him forward: he trusts the leader and the established route and therefore "grimly followed Rob into the eerie blue labyrinth." The combination of treating the inner voice as a false alarm and relying on the guide and the group's fixed procedures leads him to continue despite his fear.