What made repentance through baptism unique? As you may be aware- the 7 Feasts that the Lord gave to the Israelites are a picture - they foreshadow or point to Christ and Christ is in all the feasts. The point here is that this is the same kind of picture or typology in baptism. The word in Greek is Baptizo, and literally means "to bury". Baptism is a picture of Christ's death, burial and resurrection and a public proclamation to the world of one's decision and commitment to Christ, and when one is baptized he is buried with Christ, -immersed under the water, and then raised with him -like Paul said.
While it's debatable if baptism ever existed before Christ, it was not part of Jewish culture. The apostles were all Jews, as were the early disciples and the early church, so Baptism was a crucial part of showing one's allegiance as a Follower of Yeshua/ Christ. Remember that Paul who wrote 2/3 of the New Testament and the verse that says be baptized for the forgiveness of your sins was in a Jewish context to a Jewish audience. It doesn't save us.
Secondly, but also very important. In the old testament sackcloth and ashes were a sign of mourning primarily, and also repentance, but this was the custom when someone died, and there was nothing at all to do with repentance. So sackcloth and ashes had different roles/ dual purpose. In contrast, baptism has nothing to do with mourning, but only an act of obedience- the outer sign/ proclamation of an inner work - namely the public profession of a person's faith in Christ, and this always followed conversion.
Some denominations practice infant baptism, but they take verses out of context to support this. There are passages that say for example "Simon and his whole household" were baptized, and extrapolate that to assume babies. But in scripture, baptism always follows a conscious decision to follow Christ. For early Christians, this was a very very serious commitment an even cost some their lives, as it does today in some countries like China.