Albert Barnes on Acts 25:4

But Festus answered, etc. What induced Festus to refuse their request, is not known. It is probable, however, that he was apprized that Paul was a Roman citizen, and that his case could not come before the Jewish sanhedrim, but must be heard by himself. As Cesarea was also at that time the residence of the Roman governor, and the place of holding the courts, and as Paul was lodged there safely, there did not appear any sufficient reason for removing him to Jerusalem for trial. Festus, however, granted them all that they could reasonably ask, and assured them that he should have a speedy trial.