Why was the Bill of Rights added to the Constitution almost immediately after the Constitution was ratified?
While there was real debate and differences at the Constitutional Convention about what the powers of the government would be, or how that power would be limited, there wasn't nearly as much disagreement about a Bill of Rights. For Anti-federalists, this was a dealbreaker. They could not support a government that did not enshrine certain basic protections for its citizens, based largely on what they viewed as injustices under the rule of Britain.
The problem was that by 1789 the debate had dragged on for almost two years, and the delegates were afraid, rightly so, that they might not be able to agree on a Constitution and the convention would break up without accomplishing anything.
So they came to an understanding that a Bill of Rights would be a first priority after the Constitution was ratified and the government was in place. They stayed true to their word and the Bill of Rights was approved and added in 1791.