Because the buck stopped with Pilate. He was ultimately in charge as the Roman governor. He controlled all of the military forces in the region. He could do pretty much whatever he wanted. He could have spared Jesus if he really wanted. But like Peter (who denied Jesus three times), he was weak willed and caved in to the mob.
It seems to me that Caiaphas was evil, and Pilate was the victim of a no-win situation that the high priests set up... so it bothers me that Pilate gets thousands of years of scorn, and Caiaphas skates. The soldiers were acting beyond Pilates orders, so it still seems like Pilate is a victim, not a monster.
Pilate deserves more censure IMO than does Caiaphas. Caiaphas acted in the way he thought was right, which was to condemn someone he saw as a heretic and a threat to the religious order. Pilate knew Jesus was not guilty of anything worthy of death, but he gutlessly ordered that the sentence be carried out. It would be like seeing a judge today finding someone innocent, but ordering his execution anyway because "the crowd got ugly".