Why did John the Baptist preach repentance through water baptism and specifically how is that different than repentance through ashes and sackcloth?

The act of wearing sackcloth and ashes was a sign of mourning, sometimes as a sign that one was so repentant that they mourn. Mordecai wore sackcloth and ashes because of the decree to wipe out the Jews (Esther 4:1). His actions was completely unrelated to repentance as far as we know. This was done multiple times as a sign of mourning. The citizens of Nineveh did this as a sign of repentance and mourning.

The Baptism of John was a baptism of repentance. Today, baptism using water is a way to tell others that you are a Christian.

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.

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.