Maybe it was just knowledge of this practice that John saw as a way of proclaiming his message and getting people to commit themselves to change their lives in preparation for the coming Christ. John the Baptizer appeared in the wilderness, proclaiming a baptism of repentance for the forgiveness of sins. Mark tells us a lot in one short sentence. Notice John comes proclaiming. There is nothing apologetic about his approach.
Proclamation demands attention and reaction-negative or positive.
Notice, too, how the idea of baptism is mentioned twice. To say Yes to such an invitation is a very significant decision. It commits one to act very publicly and to reveal a great deal about one's private life. People came to John from all over and in great numbers. If you chose to move out from the crowd of onlookers, likely there was someone who knew you and what you'd done in your life, everything.