Gomez was getting progressively more dominant, and would have probably taken Castillo out badly in a round or two had Castillo(or his corner) not called it prior to Round 6. Castillo got rocked a bit at the end of Round 5 in particular. It was a good call from his corner as Castillo just wasn't punching much, was getting beat up, and didn't affect Gomez with the few punches he did land, so a miracle comeback wasn't going to happen.