Because of the violence of the crowd
Then the tribune came up and arrested him and ordered him to be bound with two chains. He inquired who he was and what he had done. Some in the crowd were shouting one thing, some another. And as he could not learn the facts because of the uproar, he ordered him to be brought into the barracks. And when he came to the steps, he was actually carried by the soldiers because of the violence of the crowd, for the mob of the people followed, crying out, "Away with him!"
(Acts 21:33-36 ESV)