This is the explanation given in the Catholic Commentary on Sacred Scripture:
Thus Acts ends on a triumphal note. The final two words in Greek are literally “boldness” and “unhindered.” Although a prisoner, Paul continues to proclaim the gospel with complete assurance and without hindrance. Even if the messenger is chained, the message is not (see 2 Tim 2:9). Here lies the reason why Luke does not tell us what happened to Paul at the end of the two full years. Though Luke knows what happened to Paul, his narrative is not primarily about Paul but about the spread of the good news of Jesus Christ.
Luke ends his two-volume work on a high note: Paul is proclaiming the kingdom and teaching about the Lord Jesus Christ “with complete assurance and without hindrance.” This open-ended conclusion implies a challenge to us, his readers. The examples of Peter, John, Stephen, Philip, Barnabas and Paul summon us too to be daring witnesses to Jesus. It is now up to us to continue the proclamation of the gospel boldly and without hindrance, no matter what obstacles we may encounter.