Albert Barnes Commentary Acts 25:4

"Howbeit Festus answered, that Paul was kept in charge at Caesarea, and that he himself was about to depart [thither] shortly." — Acts 25:4 (ASV)

But Festus answered, etc. What led Festus to refuse their request is not known. It is probable, however, that he was aware that Paul was a Roman citizen, and that his case could not come before the Jewish Sanhedrin but must be heard by him. Caesarea was also the residence of the Roman governor at that time and the place where courts were held. Furthermore, Paul was securely lodged there. Consequently, there did not appear to be any sufficient reason to move him to Jerusalem for trial. Festus, however, granted them all that they could reasonably ask and assured them that Paul would have a speedy trial.