What made repentance through baptism unique?
Baptism existed prior to John in the conversion of someone to Judaism and in the ordination of priests. Calls for repentance had existed before John particularly before the Babylonian captivity.
We have from the dead sea scrolls a picture of ascetics forming a religious community apart from what they considered an apostate or secular Israel from a century before to possibly overlapping with John’s time.
The combination of a call to repent (change one’s mind) and take seriously one’s heritage, obligations to God, and course of one’s life and a symbolic cleansing indicative of putting away the old life and becoming faithful to observe the law was an appeal to the individual to make a decision about how he lived his life and to make a public demonstration reflecting his desire to change.
Baptism was symbolic and declarative more than efficacious. It was also preparatory.
John 1:23 He said, I am the voice of one crying in the wilderness,
Make straight the way of the Lord, as said the prophet Esaias.
What was being "made straight" were the hearts of men called to repent.