Commentaries:
John the Baptist makes it clear that, though he was a prophet, he was not the Prophet, but rather was sent as a preparer of the way for the One who was the Prophet. John the Baptist's ministry was "unto repentance." He came revealing to people their sins, something the prophets always did. What we have here is not so much his ministry as the unique position he was in, chosen by God to be the one who would introduce the Messiah to the Hebrew world.
John W. Ritenbaugh
Prophets and Prophecy (Part One)