Robert Price offers one sensible scenario. Peter’s state of mind is the key. The disciples had expected Jesus to set up a kingdom on earth, and this did not happen. He was killed. They then expected Jesus to return, and this did not happen. Nothing was going right and this created a cognitive dissonance. Peter, who had promised loyalty to Jesus and then denied him publicly a few hours before the crucifixion, must have been feeling horrible. (The day after “Good Friday” is called “Black Sabbath,” the day the disciples were in mourning and shock.)
