Why did the framers agree to add a bill of rights to the Constitution after it was ratified?

The Bill of Rights was not a Slam Dunk. The Federalists felt the constitution as it stood protected the individual rights of people and the rights of the states and the Bill of Rights was not needed.

The Federalists included people like John Jay, Alexander Hamilton and James Madison.

The anti Federalists of course believe exactly the opposite. They wanted to nail down exactly what rights people had. James Monroe, Samuel Adams, Richard Henry Lee (from Virginia) were among the leading Anti Federalists.

The anti-Federalists apparently carried the day.

they thought it was neccesary to safegaurd individual liberty without constituitional assumtion. (federalists)