John 19:12

Upon this Pilate sought to release him: but the Jews cried out, saying, If thou release this man, thou art not Caesar's friend: everyone that maketh himself a king speaketh against Caesar.

This was vicious political blackmail. If Pilate would not do their will, they would prefer charges against him before Caesar, charges which both Pilate and themselves knew to be false; but also known to both was the fact that such charges, whether true or false, could blast the procurator out of office. Such was the political climate of the times; and, alas, it must be hailed as the usual political climate of all times. This did it. Pilate moved at once to crucify the Lord, caving in completely before the unscrupulous scoundrels before him.