Why do Protestants believe that Sacred Scripture is the only source of Divine Revelation?

Protestants usually don't think the Bible is the only source of divine revelation. The Bible itself gives other sources. Read Psalm 19 and Paul's' letter to the Romans chapter 1. Nature is one way of God to reveal himself. He also speaks through the Holy Spirit to our consciences. He may sometimes speak through prophets whose words are not part of the Bible. But these messengers must speak in accordance with the Bible, otherwise they should not be accepted.