Why did the framers of the Constitution address the legal treatment of the accused in so many amendments?
This was put in place to protect our freedoms. In England it was not uncommon to be persecuted for speaking out against the church or the Royal family. I think they wanted to be certain that the same thing could not happen here.
The Anti-federalists were very suspicious at the Constitutional Convention. Many of them believed that the whole idea of a new form of government with a stronger central authority was just a power grab that was going to devolve at some point into a monarchy. So they wanted guarantees, in the form of a Bill of Rights that would limit the power of the government, especially in specific terms as to what it could do to its citizens legally. They really valued their independence and personal freedom, and realized that if someone ever tried to take all power in the country, they would be the first targets for authorities. So they exacted guarantees, in writing, in the first ten amendments that very specifically limited what government could do to the legally accused so that no gray area or vagueness existed. The promise of such a Bill of Rights is what won their support for the Constitution and probably assured its ratification.
This is because the Framers were really concerned with the government using criminal procedures to put people in jail for political purposes. They wanted to ensure that this would not be possible.
Under English rule, it was more possible for the government to convict people without fair trials. This could be done simply to silence political opposition. Political opponents could be jailed without having a jury trial or without being represented by a lawyer, for example. The Americans felt that this was too much of a danger to their liberties and so they wanted to make sure it could not be done in their new country.